White slows just the reds by 50% for 5s. When it is active and you get another white, you refresh the 5s timer. Also, as time passes, the speed range of the reds decreases, but they spawn more frequently.
Green doesn't stack, so getting another when you have the shield does nothing.
Yellow gives you different extra points depending on how long you've been playing. Longer playing = More points.
I've also been adding additional power ups for future versions like shrinking your player size for 5s to make it easier to dodge in the later stages. :P