Great start to a fun game! I have two ideas for the game. One of them is to have a key in the game for which color each button does. I found that I would have to press each button several times because I couldn't remember which color went to which button. Alternatively, In order to simplify controls even more, you could make a single button which toggles between the three colors and have an on-screen indicator to show which color is next. This would be my preferred option since there are only three colors and cycling between them would be fast enough for the game. The second thing is I would alternate between fast waves and slow waves to follow a three act story progression. I think in some game engines, you may be able to do this by literally drawing a curve in the inspector so you have super fine control of the way that the waves spawn.