Great game
I liked it very much. Concept is as far as i know totally original, and the game is very simple and addictive.
An idea that is meant to be constructive: Sometimes the collision detection seems to happen 1 frame before the collision between the graphic, even if is not so.
My advice is to see how would come if the program "paints" the background red 1 frame after collision detection. more complex way to give idea collision happened could be to add a "pop" or fading away animation.
anyway the bad ball doesn't modify theyr behavior while bumping a growing good ball.