This was a really interesting concept - in some ways the closest thing I can think of I've ever come across is the Logo programming language, and its turtles: particularly, roller up/roller down made me think of the turtles' pen up/pen down. That gets you brownie points in my book, Logo is nice.
If anything, I think the combo of the physics modelled driving and roller, the worry about paint running out, and the order combined to make the game a bit too hard (for me, at least!).