Oh wow, this was more addictive than Flappy Bird. I really like the game. I want to say that you should add more stuff to it, but I'm afraid that it would ruin the simplicity of the game. I don't know to tell you the truth!
This is where I get a little nitpicky. I would have liked it if the bullets changed color the moment AFTER I pressed WASD, not until after the dies color appears face up. Also it is kind of easy to exploit the game when you camp at one end of the screen and wait for the enemies to appear. Then again, maybe that's just a strategy.
That's pretty much it. My high score was 127.