A colleague from work showed me another painting tool for the iphone. It's called 'brushes' and for my taste it feels even better than painting with 'colors'. The undo function is faster and it supports variable zoom levels. I have the feeling that it's more stable than 'colors' too. 'colors' freezed a couple of times but I didn't have trouble with 'brushes' yet. You have to change the opacity of the brush manually. I guess it's a matter of taste. Both are totally worth the 5 bugs.
Here is a little test with 'brushes'.