The slight mouse and volume issues have been pointed out so I won't harp on about them.
I would like to add though that the music felt out of place in the gameplay- I would have preferred either silence or something more dramatic and classically Morricone to really nail the vibe of old western movie shoot-outs. The gunshot noise is also a bit generic and in your face and some sound effects for exploding barrels/killing enemies would have been some nice juice.
Gameplay wise, it's *really* good. The three second wait serves the double function of letting you get a bead on the level and of building tension. While it could be taught to the player a little better, the Purple > Yellow > Red thing is smart and could built upon in an actual game.
Speaking of, thinking about how this could be developed further gets me pretty excited. There are tons of possibilities that I'm sure you've already thought about so I won't just list them like I'm being visionary.
Overall, really great work! Good luck in the results.