At first you will be filling in blank bubbles with colour, but soon you will be adding colours to other colours, blending them together in order to complete levels. That means you will need to be thinking ahead that much more, and you will need to know which bubbles will create what colours. Of course you can just, you know, figure it out, if you are good with that sort of thing. If you're more like me, however, you can instead hold a bubble instead of tapping it to see what colour you will get.