I don't have much to say that hasn't already been said. You used the limitation in a creative way and it truly feels like it is an essential part of the game.
Having feedback for hitting the targets would be nice (a sound or enemy flashing white). The bullet speed also felt a bit low, I got overwhelmed a few times by the enemies ahah.
Something you could improve if you intend on picking up the game again, would be introducing the mechanics of each color one at a time instead of the big tutorial at the beginning.
Good job on the awesome game and keep it up!