Have you thought about maybe having a horizontal canvas and have the bot = roll around on it while painting ? Then the wheels could be part of the = art creation hardware. Spongey aircraft wheels; wide wheels; narrow = wheels, etc.
Also, I thought that if you incorporated an infrared sensor into the = brush / brush-holder, then you could have the bot make decisions based = on the paint already laid down on the canvas, since different colors = would reflect differently. For example, put some red paint next to a = blue blob.
Just some thoughts on it ... JCD