Thanks for playing! The other balls unlock as your number of bounces gets higher. It might take a little too long for a jam game though (second ball drops at 1500). I could probably speed up the first couple unlocks to make what's happening more obvious.
I'm honestly still a little shocked that this implementation works at all in browser. The signed distance field calculations are just fast enough to remain playable at full resolution, although I do wish I could have made the eraser a bit bigger.