oh well that's Photoshop :) It can be done with mouse. the method is actually pretty low tech, you put your basic colours on and then you use the Hard Round brush (the basic setting of Paintbrush tool) and blend it over and over. Like this www.tangwistel.com/blend_tut.shtml
I did most of it that way and then added bits of yellow where the light would go for the reflection type thing.
...oh, and bubbles are a cheat, I drew them flat and then copied the layer and blurred it a bit >_>
like the coloring!
favs!