How colours mix

Stirred paint lands between its two colours; washes laid over each other go darker

A screen makes colour by adding light. Red light plus green light looks yellow. Paint works the other way round. Each pigment soaks up part of the light, and what's left bounces back to your eye. Yellow paint soaks up blue light, blue paint soaks up red, and a mix of the two lets mostly green through.

Professor Oops keeps colour the paint way round. Every spot of paper holds three numbers: how strongly the colour there absorbs red, green and blue light. Those numbers are turned back into a colour only when the page is drawn.

That one choice gives two kinds of mixing for free, depending on where the colour lives. Colour soaked into the paper stacks. A wash over a dry wash adds its absorbance to the first, so the light has to get through both and comes back darker than either, like a glaze. Paint, ketchup and other thick stuff lying on top is stirred. Each spot of it stores its colour multiplied by how much of it there is, so when two blobs run together their colours average by volume. Nobody has to tell them to mix.

Two chosen colours shown mixed four ways: a screen-style RGB average, stirred as paint, layered as washes, and layered washes built up to the game's cap — Try it: mix two colours four ways The game's own rule
Pick two colours from the game's palette and slide how much of the second goes into the stir. **What to look for:** blue and yellow stirred come out green, where a screen's usual average comes out a dull grey-green. Laid over each other they go darker than either. (Layered lays one full unit of each colour's absorbance; built up takes two washes to the wash rule's ceiling.) Then try black and white: stirred, it stays dark, because this model has no scattering white (see where we simplify).

In the game, a heap of yellow tube paint and a heap of blue, stirred into green with a palette knife where they meet, beside a yellow and a blue watercolour wash crossing into a darker, duller green — **Two laws on one page.** Tube paint stirred with the knife lands between its colours. Two watercolour washes crossing go darker than both.

Washes laid over each other go darker than both; stirred paint lands in between

Watercolour washes laid over each other read 0.183, darker than yellow (0.670) or blue (0.245) alone; stirred tube paint reads 0.479, between its 0.814 and 0.355.

watercolour, one wash over the other
tube paint, stirred with the knife
paint brush, strokes crossed

Data table

Washes laid over each other go darker than both; stirred paint lands in between
	watercolour, one wash over the other	tube paint, stirred with the knife	paint brush, strokes crossed
yellow alone	0.67	0.81	0.81
blue alone	0.25	0.36	0.35
both together	0.18	0.48	0.58

Source: Measured on the released engine. Automated tests check that stirred blue and yellow come out green, not grey.

Each tool lays colour its own way, so a scribbled patch never turns to mud

Dab real watercolour on the same spot again and again and it gets deeper, but it never turns black. In the game each drawing tool has its own rule for laying colour, and every rule levels off. A wash creeps up to a ceiling set by its own colour. A marker saturates instead of getting darker forever. Pencil levels off at a strength set by its colour and how hard it's pressed, and crayon stops adding once the spot is as dark as its own colour. So a patch a child goes over twenty times gets deeper but stays recognisably the colour the child chose.

The price is small and honest. Two thin glazes multiply almost exactly. Two washes built all the way up don't: each channel ends at the darker of the two ceilings.

A wash levels off at its own ceiling; plain adding would run on toward blackOne stroke lays several stamps on a spot. The blue channel levels off at yellow's ceiling and the red at blue's; the plain sums run off the top of the chart.

After 60 stamps of yellow the blue channel levels off near 3.15; after 60 more of blue the red channel ends near 2.92, while the plain sums run past 6.

red channel
green channel
blue channel
red, plain sum
green, plain sum
blue, plain sum

Data table

A wash levels off at its own ceiling; plain adding would run on toward black
stamps on one spot (60 of yellow, then 60 of blue)	red channel	green channel	blue channel	red, plain sum	green, plain sum	blue, plain sum
0	0	0	0	0	0	0
1	0.002	0.014	0.184	0.002	0.014	0.184
2	0.003	0.027	0.358	0.003	0.028	0.368
3	0.005	0.041	0.522	0.005	0.042	0.553
4	0.006	0.053	0.676	0.006	0.056	0.737
5	0.008	0.066	0.822	0.008	0.07	0.921
6	0.01	0.078	0.96	0.01	0.084	1.105
7	0.011	0.089	1.089	0.011	0.098	1.289
8	0.013	0.101	1.212	0.013	0.112	1.474
9	0.014	0.112	1.327	0.015	0.126	1.658
10	0.016	0.122	1.436	0.016	0.139	1.842
11	0.017	0.133	1.539	0.018	0.153	2.026
12	0.019	0.143	1.635	0.019	0.167	2.21
13	0.02	0.153	1.727	0.021	0.181	2.395
14	0.022	0.162	1.813	0.023	0.195	2.579
15	0.023	0.171	1.894	0.024	0.209	2.763
16	0.025	0.18	1.971	0.026	0.223	2.947
17	0.026	0.189	2.043	0.027	0.237	3.132
18	0.028	0.197	2.111	0.029	0.251	3.316
19	0.029	0.206	2.176	0.031	0.265	3.5
20	0.031	0.213	2.236	0.032	0.279	3.684
21	0.032	0.221	2.293	0.034	0.293	3.868
22	0.033	0.229	2.347	0.036	0.307	4.053
23	0.035	0.236	2.398	0.037	0.321	4.237
24	0.036	0.243	2.446	0.039	0.335	4.421
25	0.038	0.25	2.491	0.04	0.349	4.605
26	0.039	0.256	2.534	0.042	0.363	4.789
27	0.04	0.263	2.574	0.044	0.377	4.974
28	0.042	0.269	2.612	0.045	0.391	5.158
29	0.043	0.275	2.648	0.047	0.404	5.342
30	0.045	0.281	2.682	0.048	0.418	5.526
31	0.046	0.287	2.714	0.05	0.432	5.71
32	0.047	0.292	2.744	0.052	0.446	5.895
32.57						6
33	0.049	0.298	2.772	0.053	0.46
34	0.05	0.303	2.799	0.055	0.474
35	0.051	0.308	2.824	0.057	0.488
36	0.053	0.313	2.848	0.058	0.502
37	0.054	0.318	2.871	0.06	0.516
38	0.055	0.322	2.892	0.061	0.53
39	0.057	0.327	2.912	0.063	0.544
40	0.058	0.331	2.931	0.065	0.558
41	0.059	0.336	2.948	0.066	0.572
42	0.06	0.34	2.965	0.068	0.586
43	0.062	0.344	2.981	0.069	0.6
44	0.063	0.348	2.996	0.071	0.614
45	0.064	0.351	3.01	0.073	0.628
46	0.065	0.355	3.023	0.074	0.642
47	0.067	0.359	3.036	0.076	0.656
48	0.068	0.362	3.047	0.078	0.67
49	0.069	0.365	3.059	0.079	0.683
50	0.07	0.369	3.069	0.081	0.697
51	0.071	0.372	3.079	0.082	0.711
52	0.073	0.375	3.088	0.084	0.725
53	0.074	0.378	3.097	0.086	0.739
54	0.075	0.381	3.105	0.087	0.753
55	0.076	0.384	3.113	0.089	0.767
56	0.077	0.386	3.121	0.091	0.781
57	0.079	0.389	3.128	0.092	0.795
58	0.08	0.392	3.134	0.094	0.809
59	0.081	0.394	3.14	0.095	0.823
60	0.082	0.396	3.146	0.097	0.837
61	0.247	0.453	3.146	0.267	0.914
62	0.403	0.508	3.146	0.436	0.992
63	0.55	0.559	3.146	0.606	1.069
64	0.688	0.608	3.146	0.775	1.147
65	0.819	0.654	3.146	0.945	1.224
66	0.942	0.698	3.146	1.115	1.301
67	1.059	0.739	3.146	1.284	1.379
68	1.169	0.779	3.146	1.454	1.456
69	1.272	0.816	3.146	1.624	1.534
70	1.37	0.852	3.146	1.793	1.611
71	1.463	0.885	3.146	1.963	1.688
72	1.55	0.917	3.146	2.132	1.766
73	1.632	0.948	3.146	2.302	1.843
74	1.709	0.976	3.146	2.472	1.921
75	1.783	1.004	3.146	2.641	1.998
76	1.852	1.03	3.146	2.811	2.075
77	1.917	1.054	3.146	2.981	2.153
78	1.978	1.077	3.146	3.15	2.23
79	2.036	1.1	3.146	3.32	2.308
80	2.091	1.12	3.146	3.489	2.385
81	2.143	1.14	3.146	3.659	2.462
82	2.191	1.159	3.146	3.829	2.54
83	2.237	1.177	3.146	3.998	2.617
84	2.281	1.194	3.146	4.168	2.695
85	2.322	1.21	3.146	4.338	2.772
86	2.36	1.225	3.146	4.507	2.849
87	2.397	1.24	3.146	4.677	2.927
88	2.431	1.254	3.146	4.846	3.004
89	2.464	1.267	3.146	5.016	3.082
90	2.494	1.279	3.146	5.186	3.159
91	2.523	1.291	3.146	5.355	3.237
92	2.55	1.302	3.146	5.525	3.314
93	2.576	1.313	3.146	5.694	3.391
94	2.6	1.323	3.146	5.864	3.469
94.8				6
95	2.623	1.332	3.146		3.546
96	2.645	1.341	3.146		3.624
97	2.665	1.35	3.146		3.701
98	2.685	1.358	3.146		3.778
99	2.703	1.366	3.146		3.856
100	2.72	1.373	3.146		3.933
101	2.736	1.38	3.146		4.011
102	2.751	1.386	3.146		4.088
103	2.766	1.393	3.146		4.165
104	2.779	1.399	3.146		4.243
105	2.792	1.404	3.146		4.32
106	2.804	1.409	3.146		4.398
107	2.816	1.414	3.146		4.475
108	2.826	1.419	3.146		4.552
109	2.836	1.424	3.146		4.63
110	2.846	1.428	3.146		4.707
111	2.855	1.432	3.146		4.785
112	2.864	1.436	3.146		4.862
113	2.872	1.44	3.146		4.939
114	2.879	1.443	3.146		5.017
115	2.886	1.447	3.146		5.094
116	2.893	1.45	3.146		5.172
117	2.9	1.453	3.146		5.249
118	2.906	1.456	3.146		5.326
119	2.911	1.458	3.146		5.404
120	2.917	1.461	3.146		5.481

Source: Arithmetic from the game's watercolour rule at full brush, with the palette's yellow and blue.

Water makes marker run but leaves pencil and crayon where they are

Pour water over a real drawing and the marker runs while the pencil and crayon stay put. Marker ink is dye, and dye dissolves. Pencil and crayon are solid pigment pressed into the paper.

The game keeps the same difference. Beside its three colour numbers, every spot records how much of that colour is dissolved dye and how much is held in the fibres. Water may move only what isn't held, and dye is never held. Each tool says what it lays. Marker ink is all dye. Watercolour is part dye and part pigment grains. Coloured pencil, grey pencil and crayon are almost entirely held.

So a wash brushed over a pencil drawing blooms right through, and the line is still there when it dries. Pour water on pencil or crayon and over 95 % of the line stays, which an automated test checks. Rub the same pencil line and it smudges, because graphite sits loose on the fibres. Crayon hardly smudges.

One spot of paper cut open, from the top layer down

Floating film

Oil, marbling ink.

Moves on the water's surface.

Goop

Paint, ketchup, mud. Its colour is kept together with how much of it there is.

Flows on its own, stains the paper below.

Water

Standing water and wetness in the fibres.

Carries whatever is free.

The paper itself

All the colour soaked in, split into:

Dye: moves with water, each dye at its own speed.
Free pigment: the rest of the colour. Moves, more slowly.
Held: water can't move it.
Loose: a rubbing finger can.
Wax, soap: keep washes and bleach off.

Marker is all dye; pencil and crayon are almost all held

Marker is 100 % dissolved dye and watercolour half dye, half free pigment; coloured pencil holds 98 %, grey pencil 97 % and crayon 99 % in the fibres.

dissolved dye (runs)
free pigment (moves slower)
held in the fibres (stays)

Data table

Marker is all dye; pencil and crayon are almost all held
	dissolved dye (runs)	free pigment (moves slower)	held in the fibres (stays)
Marker	100	0	0
Watercolour	50	50	0
Coloured pencil	0	2	98
Grey pencil	0	3	97
Crayon	0	1	99

Source: The shares each drawing tool lays in the game.

A marker dot splits into rings because its dyes travel at different speeds

Draw a black dot, drop water on it and wait. A blue ring spreads out, a pink band follows, and a yellow patch stays in the middle. Science teachers call this chromatography. Black ink is a mix of dyes, and each one clings to the paper fibres by a different amount.

In the game a dye is one of the three colour channels. The dye that soaks up red light is the cyan one, green's is the magenta, blue's is the yellow. Whenever water moves between two spots of paper, each dye goes along at its own share of the water's speed. In a black marker the cyan dye nearly keeps up with the water, the magenta lags well behind and the yellow hardly moves. The water code has no idea what a marker is, and nothing draws a ring. The rings come out of three speeds.

The speeds were chosen so the rings show clearly, so they are tuned, not measured. In the released game every marker splits with black's three speeds. next update Each ink gets its own: green splits into yellow outside blue, purple into pink outside blue, brown into yellow, pink and a little blue.

A marker dot on paper, with water spreading from it and each colour of dye travelling at its own speed — Try it: a marker dot in a drop of water The game's own rule
A drop has just landed on a black marker dot. Tap the paper to add another drop, or press Add a drop. **What to look for:** the cyan dye rides out with the water's edge and dries there as a blue ring, the magenta stops halfway, the yellow hardly leaves the dot. The readout gives how far out each dye sits on average and checks none is lost. The water's spread, the dye rule and every rate are the game's; the drop soaking in and the drying are simplified. Green, purple, brown and red show each ink's own split next update.

A black marker dot under a drop of water in the game, opened into a blue ring, a pink band and a yellow centre. — **The real thing, in the game.** A black marker dot a minute after a drop of water.

A watercolour wash stays its own colour right up to its darker rim

A wash's dye has no reason to split. In a wash, free dye goes at the same speed in all three channels, so a blue wash stays blue to its edge. In an automated test, every spot of a blue, red or yellow wash 30 seconds into drying is within 3° of hue of the middle.

The pigment half is grains, and grains only ride along while the paper is properly wet. As a wash dries, its thin edge empties first, water flows out toward it and brings colour with it. In the game's test a dried wash's rim is 1.3 to 2.5 times as dark as its middle, as in a dried coffee drop ^{Deegan 1997}. The picture of water in the sheet carrying pigment to the edges comes from early computer watercolour ^{Curtis 1997}; the game gets there with a much simpler local rule. Wet a dried wash again and it blooms in its own colour.

The rim of a drying wash grows darker than its middle

A watercolour dab goes from 1.02 at 20 s to 2.54 at 180 s; an ink drop from 1.44 at 20 s to 4.15 at 60 s.

watercolour dab
ink drop

Data table

The rim of a drying wash grows darker than its middle
seconds after laying it	watercolour dab	ink drop
20	1.02	1.44
60	1.15	4.15
180	2.54

Source: Measured on the released engine. An automated test holds a dried wash's rim to 1.3 to 2.5 times the inside.

Dye moves in damp paper; pigment grains need it wet

Free dye is carried at its full 0.9 share from a wetness of 0.2; pigment grains reach their 0.5 share only at a wetness of 0.5.

free dye
pigment grains

Data table

Dye moves in damp paper; pigment grains need it wet
wetness of both cells	free dye	pigment grains
0	0	0
0.02	0.09	0.0008
0.04	0.18	0.0032
0.06	0.27	0.0072
0.08	0.36	0.0128
0.1	0.45	0.02
0.12	0.54	0.0288
0.14	0.63	0.0392
0.16	0.72	0.0512
0.18	0.81	0.0648
0.2	0.9	0.08
0.22	0.9	0.0968
0.24	0.9	0.1152
0.26	0.9	0.1352
0.28	0.9	0.1568
0.3	0.9	0.18
0.32	0.9	0.2048
0.34	0.9	0.2312
0.36	0.9	0.2592
0.38	0.9	0.2888
0.4	0.9	0.32
0.42	0.9	0.3528
0.44	0.9	0.3872
0.46	0.9	0.4232
0.48	0.9	0.4608
0.5	0.9	0.5
0.52	0.9	0.5
0.54	0.9	0.5
0.56	0.9	0.5
0.58	0.9	0.5
0.6	0.9	0.5
0.62	0.9	0.5
0.64	0.9	0.5
0.66	0.9	0.5
0.68	0.9	0.5
0.7	0.9	0.5
0.72	0.9	0.5
0.74	0.9	0.5
0.76	0.9	0.5
0.78	0.9	0.5
0.8	0.9	0.5
0.82	0.9	0.5
0.84	0.9	0.5
0.86	0.9	0.5
0.88	0.9	0.5
0.9	0.9	0.5
0.92	0.9	0.5
0.94	0.9	0.5
0.96	0.9	0.5
0.98	0.9	0.5
1	0.9	0.5

Source: The shape of the game's own rules for what the water carries.

Red cabbage juice works as a real acid test, turning pink with vinegar and blue-green with baking soda

Red cabbage juice is a real acid detector. Lemon turns it pink, baking soda turns it blue and then green. In the game, a spot soaked with cabbage juice stores no colour for it at all. It stores how much dye is there. Each time that part of the page is packed for the screen, the game reads how acidic the spot is and looks the colour up in a chart.

The chart has 141 shades, one for every tenth of a pH unit from 0 to 14. They are blended from 8 hand-picked colours, in the order published cabbage charts give. The shades are ours, so we say the colour is looked up from a chart, not computed from chemistry.

Because the colour is never stored, it can never go out of date. Add soda and the spot changes colour the next time it's redrawn. Turmeric and tea work the same way, as a shift from their own stain colour. A blob of blueberry on soap turns blue-green, because goop carrying a dye reads how far the paper under it is from neutral. An automated test checks it.

The red cabbage colour chart from pH 0 to 14, with the chosen pH marked and a large swatch of its colour on paper — Try it: the cabbage colour chart The game's own rule
Drag along the strip or use the pH slider. **What to look for:** the colour is the chart's entry times the amount of dye, so more juice gives a deeper shade from the same entry; only the pH moves you along the chart. The ticks under the strip are the 8 hand-picked knots.

In the game, paper stained purple with red cabbage juice, with a pink spot where vinegar was dropped and a blue spot round a little white heap of baking soda — **One dye, three colours.** A drop of vinegar and a pinch of baking soda on cabbage-stained paper. The blue round the soda is the next update's next update; in the released game the same soda turns it blue-green.

Bleach and sunlight fade colours unevenly, so black marker turns orange before it goes pale

Draw a bleach pen over black marker and it doesn't just go paler. It turns red-brown, then orange, then pale yellow, the way real bleach-pen art does. The game's bleach takes absorbance out faster in red and green than in blue. Black loses its yellow part last, so the in-between colours appear on their own. Crayon or soap under the colour keeps most of the bleach off, and every bit of colour it removes uses some of the bleach up, so a small drop on a dark patch runs out.

Bleach is a named rule in the game's table of reactions, so this part is written chemistry, not emergent chemistry. What it shares with everything else is that it reads the colour numbers, whatever put the colour there. Rust in the paper bleaches just like marker.

Sunlight fades too. Leave a page for a week in a sunny window and the dyes fade, reds and yellows first. Whatever lay on the paper, a sticker, tape, a leaf or a blob of paint, keeps its patch dark, and you get a sun print. In the released game the fade is three flat factors on every uncovered colour, pencil included. next update The fade depends on what made the colour. Marker dye takes the full fade, crayon and coloured pencil much less, grey pencil none, and the paper itself yellows a little.

Black, red and blue marker stripes on paper with a white crayon stripe, fading through brown, orange and yellow where bleach was dropped, except under the crayon — Try it: bleach on marker stripes The game's own rule
Black, red and blue marker stripes, with an invisible white crayon stripe drawn down the right. Two drops of bleach have just landed. Tap to drop more, or press Drop bleach. **What to look for:** black goes through brown and orange to pale yellow, thinner bleach at a drop's edge leaves an orange ring, and the colour under the crayon stays. The drop's size is ours; the fading, the bleach used up and the wax's protection are the game's rule. In the game bleach also travels with the water; here it stays where it lands.

In the game, three bleach-pen lines across a black marker patch: each fades to pale with red and orange edges, but where it crosses a white crayon stripe under the ink it stays dark red-brown — **Bleach-pen art.** Black marker fades through red and orange to pale. Where the lines cross a white crayon stripe under the ink, the wax keeps most of the bleach off and the black only turns dark brown.

Colours before and after a week in a sunny window, now and in the next update — A week in a sunny window, now and in the next update
Worked from the game's own fade rule (arithmetic, not a screenshot): each swatch is a solid patch on the game's paper colour. In the released game a week of sun applies the same three factors to everything uncovered. The next update next update fades by what made the colour and yellows the paper.

The page on screen is drawn from those numbers, not kept as a picture

The page you see isn't stored as a picture. The game keeps only the numbers: absorbance, dye, wetness, goop height and the rest. The simulation runs in a background worker. It packs the parts of the page that changed into small square tiles and hands them to the graphics chip, which turns absorbance back into colour for every pixel, lights it with the room's light and adds relief and shine.

Some things get their look only at that moment. Cabbage colour is looked up then. Grease stains, salt crust and wires get their colour then, and oil floating on water is folded into what's under it. Because these are worked out at drawing time, they can't drift out of step with the simulation.

Wet paper is drawn darker, as wet paper looks. Clear goop like honey or oil is looked through, not painted over. Its colour is the light that comes back up through it, so honey or oil over a dried ketchup stain doesn't pale it. When you save a picture, the same shader draws the page again with sharper edges and pigment settling into the paper's grain, detail that exists only in the saved picture.

From the simulation's numbers to the screen

Numbers in the worker

Absorbance, dye, wetness, goop and the rest, simulated in a background Web Worker.

Pack what changed

Only tiles that changed. Cabbage, grease, salt and wire get their colour here; floating oil is folded in.

Hand over

Handed to the main thread without copying.

Smooth

Filtered as absorbance, so cell edges blend like paint.

Shade

Absorbance back to light, wet darkening, goop over it, shadow, light, shine.

Screen

A child's painting, from a handful of numbers a spot.

Water, fire, bleach, tape and ants all act on the same colour, whoever put it there

Water, fire, bleach, smudging fingers, tape, stencils, sponges, ants and dogs all read or write the same colour channels. None of them asks who put the colour there. That's where the surprises come from: the water doesn't know about markers, and the bleach doesn't know what drew the colour it fades. Where we did write a special rule, such as bleach, iron-gall ink or the cabbage chart, it reads the shared numbers, never which tool put the colour there. Each tool's way of laying colour is its own rule, but they all write into the same numbers.

3

Numbers per spot of paper for the colour you see there, however many colours went into it.

2

Ways colours mix, stacking in paper and stirring in paint, from that one way of writing colour down.

512×384

Spots of paper on a large screen, each about half a millimetre across.

141

Cabbage shades in the colour chart, blended from 8 hand-picked colours.

30/s

Passes a second of water and colour through the wet parts of the page; calm patches go less often.

Shared: acid element → cabbage chart

Run a battery through salty water on paper soaked in cabbage juice

The paper turns pink at one clip. The circuit writes acid; the chart reads it; neither knows the other.

Electricity · pinned by a test

Shared: tannin + iron in the paper

Leave a rusty pin in damp, tea-stained paper

Blue-black ink forms where water brings the tea to the rust, slower under a puddle.

Chemistry and pH · pinned by tests

Shared: free colour + surface flow

Touch a soapy drop to a wet watercolour

The paint is pushed back into a ring, because surface flow moves whatever colour is free.

Runny, sticky and oily · pinned by a test

Shared: carried colour

Let a cat walk out of a paint puddle

Paw prints fade step by step as the paws run out of paint, by the same carrying rule balls, tyres and rollers use.

Animals with their own minds · pinned by a test

Shared: water wicking faster through salt

Touch a wet brush to a salty glue line

The colour races along the line and the crust takes it on.

Water in paper · pinned by a test

Shared: wax in the paper

Draw in crayon, then brush a wash or bleach over it

The crayon lines stay clean: wax keeps off the wash and most of the bleach.

Pencil, crayon and paper · pinned by tests

The game keeps paint mixing like paint and layers going darker, and skips the rest

Real colour is a spectrum of light meeting particles that absorb and scatter it. On a child's tablet we keep the part people recognise, that paint mixes like paint and layers go darker, and spend nothing on the rest. These are the trades we made, and what they cost.

In real life. Pigments absorb bands of a spectrum, and mixes depend on the whole spectrum

In the game. Three channels, worked on display values directly

Why that's enough. Cheap, fits a texture, runs on phones. Blue and yellow make green because of how the palette's colours were picked: its blue takes most red, its yellow most blue. Lost: colours that match in one light and not another, and true reflectance.

In real life. Paint scatters light as well as absorbing it ^{Kubelka 1931}

In the game. Absorption only. A published pigment-mixing method ^{Sochorová 2021} was considered and ruled out by its licence

Why that's enough. One number per channel, and both mixing laws are a single add or average. Lost: white is weak. Black and white stirred half and half come out about 0.28 grey, where a screen average gives 0.525. White crayon can't lighten a dark wash.

In real life. Glazes multiply exactly

In the game. Each channel creeps up to a ceiling set by the colour itself

Why that's enough. Dabs stay true and never turn to mud. Thin glazes still nearly multiply; two washes built all the way up end at the darker of the two ceilings in each channel, not at their sum.

In real life. How inks separate depends on the dyes, the solvent and the paper

In the game. Three tuned speeds per ink

Why that's enough. The rings must show. Not predictive. In the released game every marker uses black's speeds, so a yellow marker's yellow dye hardly runs.

In real life. Pigment is grains of many sizes

In the game. A share of the water's flow that grows with wetness

Why that's enough. Gives the coffee-ring rim. No particle sizes; granulation appears only in saved pictures.

In real life. Cabbage colour is chemistry

In the game. A chart of 8 hand-picked shades, in the order published charts give

Why that's enough. Reads clearly and can't go stale. No kinetics, no history: acid gone means pink gone.

In real life. Each dye fades at its own rate in light

In the game. Three factors per channel; next update one lightfastness per medium

Why that's enough. Readable: reds and yellows go first, blues last. That tendency is coded for every dye, and any cover counts as full shade, which gives crisp sun prints.

In real life. A stain moves colour from the liquid into the fibres

In the game. The paper gains colour; the goop doesn't lose any

Why that's enough. Cheap: nothing has to be booked between the two layers. So stains copy, and with the tide mark and iron-gall ink they're among the few places colour is made.

About this source

Colours mix like real paint because the game stores the light each one soaks up

Stirred paint lands between its two colours; washes laid over each other go darker

Each tool lays colour its own way, so a scribbled patch never turns to mud

Water makes marker run but leaves pencil and crayon where they are

One spot of paper cut open, from the top layer down

Floating film

Goop

Water

The paper itself

A marker dot splits into rings because its dyes travel at different speeds

A watercolour wash stays its own colour right up to its darker rim

Red cabbage juice works as a real acid test, turning pink with vinegar and blue-green with baking soda

Bleach and sunlight fade colours unevenly, so black marker turns orange before it goes pale

The page on screen is drawn from those numbers, not kept as a picture

From the simulation's numbers to the screen

Numbers in the worker

Pack what changed

Hand over

Smooth

Shade

Screen

Water, fire, bleach, tape and ants all act on the same colour, whoever put it there

Run a battery through salty water on paper soaked in cabbage juice

Leave a rusty pin in damp, tea-stained paper

Touch a soapy drop to a wet watercolour

Let a cat walk out of a paint puddle

Touch a wet brush to a salty glue line

Draw in crayon, then brush a wash or bleach over it

The game keeps paint mixing like paint and layers going darker, and skips the rest