I like your ideas. I was struggling a bit to find the right way to introduce the mechanics. Focusing on subtraction first is a great idea. And I think I'll make some enemies that can't be reduced below the base color, and some that can't be added above, so that there are easier enemies to fight. Then later I have enemies that go all the way from black to white. I can also start with a red level, a green level and then a blue level to introduce the feel of the game.
I greatly appreciate the feedback! Thanks for playing.