Edit: Updated the prototype. Now it supports a more "ecological" system and multiple-brushstroke entities.
After drawing, scroll your mousewheel down to plant what you drew. This will also change all your brushes. Scroll your mousewheel up to discard whatever you just drew. That doesn't cycle your brushes.
Because of egregious slowdown, only one entity can grow at a time. The others will grow: it's a first come, first serve queue.
You can click on an entity to delete it. This allows you to prune "ugly" mutations. However, this may turn out to be awkward, because Flash's hitTest system leaves much to be desired. :P
Next step: make multi-brushstrokes "adhere" to each other instead of drifting independently. This is also a first step towards layered entities...
Here's the flash prototype for my paint game, part two. This version has better brush variation and a basic system of evolution: when you stroke with a brush, it will grow in the background. The game makes a wild guess as to whether it's supposed to float or grow from the ground.
Still very prototypy, but I felt like sharing it. You can get some really nifty things out of it.
Obviously, there's a lot of work left to be done... but I have some good ideas.