I found Colors! Lite, which is like Colors! but doesn't let you save. After playing around a bit to figure things out, and learning how to blend colors a bit (it lets you control opacity with the tilt of the phone), I decided to try my hand at a something. I happened to have this picture of Milwaukee Harbour open, so I used that as a starting point and ended up with this:
I'm pretty pleased with my lighthouse, given that I don't really have any visual arts expetise. Brushes is tempting.
ETA: And I didn't realize that you could zoom in for detail in Colors!, so this is all done with a finger at the standard screen size.