A window illustration gives you the easiest kind of composition there is: a frame already built into the picture, with a little life sitting inside it or drifting past outside. Gouache suits this subject perfectly, because flat matte shapes and crisp edges read like a printed card from across the room, and a pane of frost or a street of snow only needs a handful of clean shapes to feel finished.
Below you will find 29 window scenes to paint, from a cat watching snow fall to a train window rushing past mountains, each one built to finish in a single sitting. Work through them in order if you like, starting simple and building confidence, or jump straight to the one that is already playing in your head.
Using these images: free for personal projects such as printing, coloring, cards, journals and gifts. Handmade items that include them may be sold up to 30 pieces per design. More than that is commercial use and is not permitted, and neither is selling or sharing the images themselves or passing them off as your own. Please ask before any other use.
Start with the easiest window scene on this list: a cat on a sill watching snow fall on a quiet street. Block in the window frame first with a chocolate brown rectangle, then drop in the cat as one simple seated silhouette, a rounded triangle body with two pointed ears, before adding the street below in flat icy blue and cream shapes.
Work largest shapes to smallest: frame, cat, street, snow. Keep the cat’s body one solid color with no fur texture, just a clean outline and maybe one eye dot, since gouache reads best as flat shapes rather than fine detail.
The scene usually goes wrong when the cat gets too fussy next to the flat street behind it. If that happens, simplify the cat back down to two or three shapes and let the falling snow carry the detail instead, dotted in with a small round brush or the end of a brush handle.
If you do not have a fine brush for the snow dots, a white gel pen works just as well once the paint is dry.
You Will Need
- white gel pen for snow dots
Palette: Payne’s Grey, Icy Blue, Cream, Chocolate Brown
Frost ferns creeping across an old window pane make a quiet, almost abstract subject, and they are far easier to paint than they look. Paint the glass as one flat icy blue rectangle first, then add the frost in white gouache on top, dragging a dry, nearly empty brush outward from the corners in thin branching lines.
Work corner by corner rather than trying to cover the whole pane evenly. Real frost gathers thickest at the edges and thins toward the middle, so leave more blue showing near the center of the glass.
This is where the dry brush trick earns its keep: load the brush with white, then wipe most of it off on scrap paper before touching the pane, so the bristles skip and split as they move. That broken line is what reads as frost rather than a painted leaf.
If your brush keeps laying down solid white instead of splitting, switch to an old, slightly worn brush, since a brand new one holds too much paint to skip properly.
You Will Need
- dry, slightly worn brush for frost texture
Palette: Icy Blue, Titanium White, Chocolate Brown, Cream
Look at a photo of real window frost before starting, you will notice it branches like fern fronds rather than sitting in neat snowflake shapes, which is the detail that sells this picture.
Move from the glass itself to the view through it with this rooftop scene. Paint the window frame as a simple border first, then fill the view with a row of rooftop triangles at different heights, each one a flat shape of cream, sage or icy blue against a soft grey sky.
Eye level matters here: keep the rooftops sitting just below the middle of the window so there is enough sky above for the chimney smoke to curl into. Paint the smoke last, in thin white or pale grey strokes that drift to one side, since smoke always leans with the wind rather than rising straight up.
Rooftops tend to go flat and lifeless if every triangle is the same height and angle. Vary the roof pitches slightly and stagger a chimney or two off center to keep the skyline interesting.
No fine brush for the smoke? A cotton swab dipped lightly in white paint and dragged sideways gives a soft, believable curl.
For more rooftop and skyline ideas beyond this one window, the city illustration ideas cover whole snowy streets in several cities.
Palette: Sage Green, Cream, Icy Blue, Payne’s Grey
This idea turns the frost itself into the subject: a girl drawing a heart in the frosted glass with her finger. Paint the whole pane as a flat, slightly uneven white-blue frost layer first, then paint the girl as a simple shape from behind or in profile, close to the glass with one hand raised.
Save the heart for last. Instead of painting it, scratch it into the wet frost layer with the wrong end of a brush or a toothpick, revealing a thin line of the icy blue glass color underneath. That single clean gesture carries the whole picture.
Figures drawn close to glass often come out stiff. Keep her shoulders slightly turned and her head tilted toward the heart rather than straight on, which makes the pose feel like a real paused moment rather than a stamp.
If scratching into wet paint feels risky, practice the heart shape on scrap paper first so your hand knows the motion before it touches the real piece.
You Will Need
- toothpick or brush handle for scratching the heart
Palette: Icy Blue, Titanium White, Cherry Red, Chocolate Brown
A bay window with cushions and a sleeping dog gives you a wider, cozier frame to fill. Paint the three angled panes of the bay first in light cream, then build the window seat with a few flat cushion shapes in sage and cherry red before adding the dog curled into an oval in the corner.
Work the dog as one continuous curled shape rather than separate legs and head, since a sleeping animal reads best as a single soft silhouette with just an ear and a tail tip breaking the outline.
The whole seat can flatten out if every cushion is the same size and color. Vary two or three cushion shapes and let one overlap the dog slightly, which pulls the eye into the corner where the dog rests.
If gouache feels too opaque for the soft cushion edges, thin it slightly with water on your palette before the final layer so the color sits a touch more gently.
Palette: Sage Green, Cherry Red, Cream, Chocolate Brown
Shift outward now to a Parisian window with a balcony rail piled with snow. Paint the tall shuttered window first as a narrow vertical shape, then add the iron balcony rail as a simple repeating pattern of thin brown lines, far easier to suggest with a few evenly spaced strokes than to draw rail by rail.
Load the snow on the rail last, in thick white gouache applied with a loaded brush rather than a wash, so it sits up off the page with real texture. Let a little of it drip down over the rail edge in two or three places for a natural, settled look.
Iron rails often end up looking like a ladder if the spacing is too even. Group the uprights in twos or threes with slightly uneven gaps, the way real wrought iron is actually forged.
If your white gouache looks thin, mix it with a touch of white gesso for extra body before painting the snow.
You Will Need
- white gesso for raised snow texture, optional
Palette: Chocolate Brown, Titanium White, Icy Blue, Cream
Light source becomes the whole idea in a lit shop window with the snowy street reflected in it. Paint the window glass as one warm cream or pale yellow rectangle to show the light spilling from inside, then paint the reflected street, a dimmer, slightly blurred version of the same rooftops and lampposts, as a thin overlay across the lower half of the glass.
Keep the interior objects, maybe a display of hats or pastries, as a few simple shapes silhouetted against the warm light, since detail inside a lit window should stay soft compared to the sharp outline of the frame itself.
The reflection is the part that sells the whole scene, and it usually fails if it is painted as solid as the real street. Thin the reflection colors with more water, or paint them a shade lighter than you think, so they sit behind the glass rather than on top of it.
No yellow ochre on hand? Mix cream with a touch of cherry red for a warm glow that works just as well.
Palette: Cream, Cherry Red, Chocolate Brown, Icy Blue
Bring the scene back indoors with a kitchen window with herbs and a view of a bird feeder. Paint the windowsill first as a flat brown ledge, then add two or three potted herbs along it, simple rounded green clusters in slightly different shades of sage so they do not blur into one shape.
Place the bird feeder in the view beyond, hanging from a thin brown line outside the glass, with one small bird shape perched on it. Keep the feeder smaller and less detailed than the herbs in front, since things closer to the viewer should always carry more weight on the page.
Potted herbs can turn into a green blob if you paint every leaf. Suggest the texture with three or four confident brush-tip touches per pot instead of outlining individual leaves, letting the gaps between touches do the work.
If you want a second small detail, add one snowflake shape on the outside of the glass to remind the eye this is winter.
Palette: Sage Green, Chocolate Brown, Cream, Icy Blue
Try a round frame next with an attic window framing a snowy hill. Circles are unforgiving in paint, so trace around a plate, lid or compass before you load any color, then paint the frame itself as a clean brown ring before touching the view inside.
Inside the circle, keep the composition simple: one sloped hill shape in cream and icy blue, maybe a single bare tree, and a pale sky filling the rest. A round window reads best with fewer elements than a square one, since the curved edge already does a lot of visual work.
Edges tend to wobble where the paint meets the traced circle. Paint slightly inside the line rather than right up to it, then go back with a thin brown line to sharpen the final edge once the inner color is dry.
If you do not have a compass, any round lid or glass from the kitchen traces a perfect circle.
You Will Need
- a plate, lid or compass to trace the circle
Palette: Chocolate Brown, Cream, Icy Blue, Payne’s Grey
Steam changes everything about how light and shapes behave, which is what makes a tram passing seen through a steamy cafe window such a satisfying ten minute study. Paint the glass first with a soft, uneven wash of pale grey-blue to suggest condensation, then drop in the tram as a simplified rectangle with two or three window lights, slightly blurred at the edges rather than sharp.
In ten minutes, that is the whole picture: steamy glass, one soft tram shape, done. Given forty minutes, add a café table shape in the foreground with a cup, and streak a few clear lines through the steam with a dry brush, as if a hand or a cloth had wiped the glass.
The tram will look stuck to the glass if every edge stays crisp. Lose at least one edge of the tram into the steam entirely, letting color fade rather than stop at a hard line.
If you want the full scene around it, the travel doodle ideas page has more train and transit windows to work from.
Palette: Icy Blue, Payne’s Grey, Cherry Red, Cream
A child with her nose pressed to the glass watching snow needs careful proportion to read correctly, since the face gets slightly squashed and pale where it touches the pane. Pencil in the head shape first, then flatten just the nose and lips area very slightly against an invisible vertical line marking the glass surface, keeping the rest of the face rounded normally.
Paint the child’s silhouette from behind or in three-quarter view first, in one flat color, then add the glass as a thin icy blue overlay across the front of her face only, with her breath fogging a small patch just below it.
Faces pressed to glass often come out looking squashed all over rather than just at the contact point. Keep the forehead and cheek rounded and only flatten the small area actually touching the pane for a believable result.
If the fogged breath patch looks too hard-edged, soften it by dabbing with a slightly damp finger or cloth while the white paint is still wet.
Palette: Cream, Icy Blue, Chocolate Brown, Cherry Red
A window box of evergreen sprigs and pinecones in the snow works as a quiet, decorative subject and a good warm-up piece. Paint the wooden box first as a simple flat rectangle, then build the sprigs from loose V-shaped brush strokes in sage green, overlapping two or three clusters rather than painting individual needles.
Add two or three pinecones as simple oval shapes with a few short brown strokes suggesting the scales, then finish with thick white gouache piled along the top edge of the box and resting in the crooks of the sprigs.
Evergreen sprigs can turn into a solid green mass if every stroke points the same direction. Angle your brush strokes in a few different directions within each cluster so the greenery has some movement to it.
If your sage green looks too flat on its own, mix in a touch of Payne’s grey for a cooler, more believable winter green.
Palette: Sage Green, Chocolate Brown, Titanium White, Payne’s Grey
Widen the view again with a cabin window with frost and a view of a frozen lake. Paint the log cabin interior frame first in warm chocolate brown, then the frosted edges of the glass creeping in from the corners, before opening up the center of the pane to a flat icy blue lake stretching to a line of dark pines.
Keep the horizon line low in the window, maybe a third of the way up, so there is plenty of pale sky above the lake and the frost at the edges has room to feel like it is closing in rather than crowding the view.
The frozen lake can look like flat blue paper if it has no surface at all. A few thin, slightly curved white lines across it, like wind-scraped ice, give it just enough texture without adding real detail work.
If you want a related winter scene, the winter forest painting ideas show more frozen lake and pine tree compositions.
Palette: Icy Blue, Chocolate Brown, Payne’s Grey, Titanium White
Pattern takes the lead in a window with lace curtains and a row of potted violets. Paint the window and sill first, then the curtains as two soft cream panels pulled to each side, suggesting the lace pattern with a scatter of small white dots or tiny crosses near the curtain edges rather than drawing an actual lace design.
Line up three or four small pots of violets along the sill, each flower just a few quick cherry red or purple dabs on a thin green stem, kept loose and varied in height so the row does not look mechanically even.
Lace detail can easily turn fussy and overworked. Stop the pattern dots well short of covering the whole curtain, since a light scatter along the edge reads as lace, while a dense pattern just reads as busy.
If cherry red violets feel too bold for this soft scene, a dusty mauve mixed from cherry red and a touch of Payne’s grey sits more quietly against the cream lace.
Palette: Cream, Cherry Red, Sage Green, Chocolate Brown
Movement itself becomes the subject in a train window with mountains and snow rushing by. Paint the window frame and a hint of the train interior first, dark and still, then let everything beyond the glass blur: mountain shapes stretched sideways with a dry, slightly dragged brush, and snow painted as short horizontal streaks rather than dots.
Keep the interior details, a headrest edge or a curtain corner, crisp and sharp. That contrast between a still, sharp interior and a smeared, rushing exterior is what convinces the eye the train is moving at all.
Mountains painted too neatly kill the sense of speed immediately. Drag your brush horizontally through the wet mountain color before it dries, stretching the edges into streaks, rather than keeping each peak a clean static shape.
If dragging wet paint feels unpredictable, practice the streak technique on scrap paper first to find how much water and pressure gives you the blur you want.
Palette: Icy Blue, Payne’s Grey, Titanium White, Chocolate Brown
Build a small story between two buildings with a neighbor waving across the street from an apartment window. Paint your own window frame close and large on one side of the page, then the neighbor’s building smaller and further back on the other side, with the neighbor as a simple waving silhouette in their lit window.
Scale carries this whole idea: keep the near window at least three or four times larger than the distant one, since that size difference is what reads as distance across a street rather than two windows side by side.
The wave itself can disappear if the arm is too small to notice. Angle the arm out from the body at a clear diagonal, away from the silhouette’s edge, so it reads instantly even at a small size.
If you want to add warmth, a single light inside the near window, painted in soft cream, frames the gesture without needing a second figure.
Palette: Cream, Chocolate Brown, Icy Blue, Cherry Red
Give the glass a sense of humor with a cat and a pigeon staring at each other through the glass. Paint the cat sitting upright on the inside sill first, a simple rounded triangle body with alert ears, then the pigeon perched just outside on the ledge, a plumper rounded shape with a small head tilt.
Face the two shapes directly toward each other, nearly nose to beak, with the glass line running straight down between them. That direct face-off is the whole joke of the picture, so keep both heads at the same height.
The scene loses its charm if either animal looks away or down instead of straight ahead. Paint the eye dots deliberately aimed at each other, even if that means adjusting the head angle slightly after the base shape is dry.
If you want a few more feline window scenes to pair with this one, the cat sketchbook ideas cover cats in several other window and garden settings.
Palette: Chocolate Brown, Icy Blue, Cream, Payne’s Grey
Light and color combine directly in a stained glass window throwing color onto a snowy floor. Paint the window itself with a few simple leaded panels, each one a different flat color: icy blue, cherry red, sage, outlined in thin chocolate brown lines like a simplified panel of real stained glass.
Then paint the floor below in cream or pale white, and lay colored light patches directly onto it that match the panel shapes above, slightly stretched and skewed as if cast at an angle. Those floor patches are the whole point of the picture, so keep their shapes clearly related to the panels that cast them.
The light patches look pasted on if they sit as flat solid color with no softness. Blend their edges very slightly into the cream floor with a barely damp brush so the color looks cast rather than painted on top.
If you do not want to design original stained glass panels, a simple grid of five or six rectangles in alternating colors reads perfectly well.
Palette: Icy Blue, Cherry Red, Sage Green, Chocolate Brown
Slow the pace down with a window seat with a book and a view of falling snow. Paint the seat and cushion first in sage or cherry red, then a simple open book resting on it, just two flat pale shapes meeting at a spine line, with no need to paint actual text.
Place the view of falling snow beyond the glass behind the book, kept paler and softer than the seat and book in front, since the eye should land on the reading nook first and the snow second.
A page left completely blank can look unfinished rather than deliberate. Add two or three faint horizontal pencil-thin lines across the open pages to suggest text without writing any actual words.
For more reading-corner window scenes, the book illustration ideas page is full of window seats and snowed-in libraries to borrow from.
Palette: Sage Green, Cream, Chocolate Brown, Icy Blue
Take the frame somewhere less polished with a garden shed window with old tools and frost on the glass. Paint the small, slightly crooked shed window first, then a loose arrangement of tool shapes hanging just inside, a rake head, a trowel, a coiled hose, each one a simple flat silhouette rather than a detailed drawing.
Overlap two of the tools slightly so they feel stored rather than lined up for display, then add frost creeping across the lower corners of the glass last, in the same dry-brush white technique used for the frost fern idea earlier.
Tool shapes can read as mysterious blobs without a recognizable silhouette. Keep the outline of each tool simple but accurate to its real shape, since a trowel’s pointed blade or a rake’s row of tines is what tells the eye what it is looking at.
If you are unsure of a tool’s shape, a quick search for a reference photo before you paint saves several minutes of guessing.
Palette: Chocolate Brown, Payne’s Grey, Titanium White, Cream
Light source changes the entire mood in a window at dawn with a pink sky over snowy trees. Keep the window frame and interior in cool, quiet tones, then let the view beyond glow with a soft gradient from pale pink near the horizon up into icy blue higher in the sky, painted while both colors are still wet so they blend at the seam.
Place a simple row of bare or snow-dusted trees along the horizon in dark silhouette against that glow, since a strong dark shape against a soft gradient is what makes early light look believable rather than just a pink wash.
Gradients can turn muddy if you work back into them once they start to dry. Mix both sky colors before you start, then lay pink at the bottom and blue at the top in one continuous pass, blending only where they meet.
If blending wet into wet feels tricky, a small travel brush with soft bristles gives you more control than a large one for this transition.
Palette: Cherry Red, Icy Blue, Payne’s Grey, Chocolate Brown
Bring in a different architecture with a tiny Japanese house window with a snow lantern outside. Paint the window as a simple grid of thin wooden dividers, a few horizontal and vertical lines over a soft paper-like cream pane, before adding the view beyond: a stone snow lantern sitting in a small garden, its round cap piled with snow.
Keep the lantern shape simple, a stacked arrangement of three or four geometric blocks, since real snow lanterns are architectural rather than decorative and read clearly even reduced to basic shapes.
The window grid can swallow the whole picture if the lines are too thick or too many. Keep the dividers thin and spaced generously, no more than a three by three grid, so the garden view stays the main event rather than the frame.
If straight grid lines wobble freehand, a ruler held lightly against the wet paint edge keeps them clean without needing to mask the area.
You Will Need
- ruler for the window grid lines
Palette: Cream, Chocolate Brown, Payne’s Grey, Titanium White
Return indoors for a dog with its paws on the sill waiting for its owner. Paint the dog’s body first from behind or in profile, with both front paws raised and resting on the sill edge, a pose that needs the chest and front legs stretched slightly taller than a normal sitting dog to read as alert rather than relaxed.
Measure the ears and head against the body before committing to final color, since a waiting dog’s head often tips slightly to one side or forward toward the glass, and that small tilt is what carries the emotion of the pose.
Paint the view beyond the glass soft and slightly blurred, an empty snowy path or driveway, since the whole point of the picture is the dog’s anticipation rather than any detail outside.
If the raised-paw pose looks awkward, reference a photo of a real dog standing on hind legs before finalizing the proportions, even a quick phone photo of your own pet works.
Palette: Chocolate Brown, Cream, Icy Blue, Payne’s Grey
Give the window a burst of unexpected color with a lemon tree in a pot catching winter sun. Paint the terracotta-colored pot first as a simple tapered shape, then build the tree from a central trunk line with a loose rounded cluster of sage leaves, and finish with four or five small yellow lemon shapes tucked among the leaves.
Place the tree so it catches a visible patch of warm light from one side of the window, a soft cream or pale yellow glow painted directly onto the leaves and pot on that side only, contrasting with cooler shadow tones on the other.
Lemons can disappear into a green mass if they sit too deep inside the leaf cluster. Let at least two or three lemons overlap the outer edge of the foliage so they catch the eye immediately rather than hiding.
If yellow gouache looks chalky straight from the tube, add the smallest touch of cream to warm it before painting the lemons.
Palette: Sage Green, Cherry Red, Cream, Chocolate Brown
Architecture takes the lead again with an arched stone cottage window with icicles hanging down. Paint the curved stone frame first, working the arch in two or three slightly varied shades of grey-brown to suggest individual stones without drawing each one separately.
Hang the icicles from the top edge of the arch last, painted as simple tapered white triangles of different lengths, thicker near the top and narrowing to a fine point. Keep most of them short with just one or two longer ones breaking the rhythm, since real icicles rarely hang in perfectly even rows.
Icicles painted too uniformly in length and spacing look like a fringe rather than ice. Vary both the length and the gap between each one, and let a couple overlap slightly for a more natural cluster.
If straight icicle edges are hard to control with a round brush, a small flat brush held on its edge gives a cleaner tapered line.
Palette: Payne’s Grey, Chocolate Brown, Titanium White, Icy Blue
Flip the viewpoint upward with a skylight full of snow above an artist’s easel. Paint the skylight frame first, a simple angled rectangle high on the page, with soft grey-white snow pressed against the glass from outside, letting only a dim, cool light filter through rather than painting a clear view.
Place the easel below and smaller, holding a canvas shape with just a hint of color on it, enough to suggest a painting in progress without needing to paint an actual finished scene within the scene.
The light falling from the skylight can look flat if it does not actually touch anything below. Add one pale cool patch of light landing on the floor or on a corner of the easel itself, which ties the upper and lower halves of the picture together.
If you want a related artist’s-corner subject, a quiet reading or sketching nook sits well alongside this one in a small window-themed series.
Palette: Payne’s Grey, Titanium White, Chocolate Brown, Cream
Add real movement with a girl leaning out of an open window to catch snowflakes. Paint the window frame first with both shutters or sashes pushed open, then the girl leaning forward through the opening, one arm extended up and out, palm turned skyward, with her weight clearly shifted forward over the sill.
Angle her torso and head at a visible lean, at least twenty or thirty degrees forward from upright, since a figure painted too straight simply looks like she is standing at the window rather than reaching out of it.
Scatter a few falling snowflakes around her raised hand last, kept larger and more spaced out near the hand itself so at least one clearly looks about to land on her palm.
If the leaning pose feels difficult to judge, try the pose yourself for a moment and notice how far your own shoulders shift forward, then paint from that sense of weight.
Palette: Cherry Red, Cream, Chocolate Brown, Icy Blue
Turn the glass itself soft and alive with a fogged greenhouse window with plants pressing on the glass. Paint the pane first as an overall pale, slightly uneven grey-green fog layer, then press in the plant shapes from behind it: large leaf silhouettes pushed right up against the glass, darker and more defined where they actually touch.
Leave a few patches of the fogged glass untouched by any leaf, since the contrast between clear fog and leaf-pressed glass is what makes the condensation believable rather than just a flat green wash.
Leaves pressed to glass can look like they are floating if their edges stay sharp all the way around. Soften the outer edges of each leaf into the fog while keeping the point of actual contact with the glass crisp and dark.
If you do not have a soft blending brush, a clean, barely damp finger works well for softening those leaf edges into the fog.
Palette: Sage Green, Payne’s Grey, Cream, Titanium White
Close the list the way it opened, with cats, but across a wider distance: two cats on facing sills of neighboring houses. Paint one cat large and close on your near window, then the second cat smaller on a distant window across the street, matching its pose loosely to the first so the two read as a quiet conversation across the gap.
Keep the color and detail level of the far cat simpler and slightly muted compared to the near one, the same distance trick used earlier with the waving neighbor, so the size and clarity difference alone tells the eye which cat is close and which is far.
The pair can look unrelated if their poses point in different directions. Angle both cats’ heads and bodies toward each other, even slightly, so the composition reads as a single connected scene rather than two separate windows pasted side by side.
If you enjoyed painting cats throughout this list, the cat sketchbook ideas and the cozy drawing ideas both have more sleepy, watchful cats to try next.
Palette: Chocolate Brown, Cream, Icy Blue, Payne’s Grey
What to Do Next
Pick one window that is already playing in your head, whether that is the cat on the sill or the heart scratched into frost, and start there on a piece of heavy illustration board. Block in the frame first, then the view, then the small life inside it, and let the next idea on this list wait for tomorrow’s sitting.
Frequently Asked Questions
What is the best paper for a window illustration in gouache?
Heavy cold press illustration board or watercolor paper of at least 300gsm works best. It handles the flat washes and layered frost or snow details without buckling, and gives crisp edges where gouache shapes meet.
How do you paint snow and frost without it looking chalky?
Thin white gouache slightly with water for frost layers, and keep thick, undiluted white only for raised snow on sills or rails. Dry brushing, where you wipe most paint off the brush before touching the page, keeps frost looking delicate rather than solid.
How do you keep window frames looking straight without a ruler?
Lightly pencil the frame lines first using the edge of a book or ruler as a guide, then paint just inside those lines. A slightly imperfect freehand edge over a straight pencil guide still reads as a clean frame from normal viewing distance.
Can these window illustration ideas work in watercolor instead of gouache?
Yes, though the flat matte look described here is specific to gouache’s opacity. Watercolor versions will read softer and more transparent, so plan for washes and layering rather than the solid flat shapes gouache allows.
What is the easiest window illustration idea to start with?
The frost fern pane or the cat on a snowy sill are the simplest, since each uses only a few large flat shapes and one dry-brush texture trick, with no complex figures or perspective to manage.