The game's pretty fun, and looks really cool. It really sells the apocalypse theme well, and I like the top-down shooter gameplay. The art and sound are both fantastically implemented as well, as they really help enhance the player experience.
Two small critiques I want to address are regarding the spawn system and the kill system. The zombies generally spawn in a suitable place, but there are times where they'd spawn on top of me, especially if I was moving around while waiting for them to spawn. As for the kill system, I see what you were going for, but I spent a little bit too long dodging zombies while waiting for the right color to spawn. I did encounter the rainbow ammo, but I assume that ammo type is rare.
Overall, the game is a well-rounded package that's well put together. The team did a great job!