"I'm painting with grass!" said my 5-year-old when she tried the app below.
Exactly!
"Can I make it red?"
I'll get right on that!
Click and drag in the box to draw. Then type r for red, b for blue, g for green. c clears it.
Update: p is for purple, as requested.
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The Processing code for this is here.
Originally written during Jer Thorp's Processing workshop.