I feel like I could zone out on this for hours, seriously the only thing I wish about this game is that there was a deluxe version of it I could buy and play on my 360. Definitely don't let the BattlePaint concept die out entirely, it's wicked fun.
This is your first submission to the site, ooh, I have a feeling I'm gonna be seeing you on the front page in the future. Your menu color scheme even matches the site, I don't know if you did that on purpose, but either way it was a clever move because it actually did make even the menu feel more natural.
As for constructive critique? I dunno, like other folks said I guess, more enemy types, maybe an upgrade system but I dunno, some games are better without it for smoothness purposes, like Geometry Wars. By the way, I got to about level 40 and I loved how the enemies would start spawning in the path I was trying to run, that was fun. I wonder how this game would be if I could use mouse to shoot but I am sure that thought crossed your mind too and it was your decision to use arrow keys. Still, mouse shooting may help the game flow smoother.
Good work, looking forward to more from you.