Option 28: watercolour. A small pigment simulation runs on paper. Water spreads and evaporates, pigment drifts to drying edges (the dark "coffee ring"), settles into the paper grain, and blooms when new water hits a damp wash. "Paint it" paints the bot stroke by stroke; "Living wash" lets it hop and leave stains. Tap the paper to drop water.

**This is simulated, not filtered.**
Each cell of a 200×150 grid tracks water, floating pigment and settled pigment, plus the height of the paper grain. Water spreads and evaporates. Pigment rides the water, then settles as it dries.

**Colours mix like real paint.**
Each pigment absorbs light, and layers multiply, so blue over rose makes violet, never grey mush.

**"Paint it" follows a painter's order in about 10 seconds:**
Shadow wash on the bot; the paper's white is the bot's white

**"Living wash":**
the bot hops across a pre-painted sheet, leaving shadow stains that shift colour (ultramarine → rose → yellow → green) and slowly fade.

**Tap the sky while it's still damp.**
You get a bloom (a "cauliflower"): the new water pushes pigment outward into a frilly dark ring. Painters fight these; artists also use them on purpose.

**Untick "Edge darkening," then New sheet.**
Washes dry flat and digital. The dark rim is what makes it read as watercolour.

**Granulation 100%.**
Pigment clumps into the paper's valleys, giving a speckled texture. Real ultramarine and sienna do this; smooth dyes don't.

**Wetness 2.0×.**
Paint stays wet longer, spreads further and bleeds into the bot's edges.

**The dark rims are physics.**
Water evaporates fastest at the edge of a puddle, so water flows outward to replace it and carries pigment along. Coffee rings form the same way.

**Watercolour has no white paint.**
Every highlight is untouched paper, which is why painters plan in advance.

**The wet sheen is real.**
Wet areas render slightly darker, just as damp paper looks darker, then lighten as they dry. That's why painters wait before judging a colour.

**Pigments are absorption recipes.**
"Ultramarine" is just "absorb lots of red, some green, little blue." Mixing adds absorptions: subtractive colour, like layered coloured filters.

**One cell's life**
```
brush ─► water ↑ floating pigment ↑ each step: water spreads, evaporates floating pigment flows toward drier cells (edge darkening) some settles: faster as it dries, more in paper valleys (granulation) colour = paper × e^−(settled + floating absorptions)
```

**Why rims form (side view)**
```
wet wash: ~~~~~~~~~~~~~~ evaporation fastest at edges flow: → → → → →│ water (and pigment) move outward dry result: ░░░░░░░░░░░░░▓│ dark rim at the edge
```

**Glazing vs mixing**
```
rose glaze + ultramarine glaze (dried layers) → luminous violet (light passes both) rose + ultramarine mixed wet → softer violet, granulates
```

**Critical thinking:**
watercolour's "flaws" (blooms, hard edges, granulation) are what make it beautiful, and the simulation needs extra code to create them. Is imperfection a feature you should design into digital tools?

**Lateral thinking:**
coffee rings, river deltas, salt flats and tea stains all come from particles carried by evaporating water. Could your bot be "painted" by spilled coffee, tea or ink on a tablecloth?

**First principles:**
why does mixing light (screens) move toward white while mixing pigment moves toward black? What is physically happening to the light in each case?

- Can this export a high-resolution painting to print?
- How do tools like Rebelle or Procreate simulate watercolour?