I’ve fannied around with the Sketches app for the iPhone/iPod touch (examples here and here) before. I gave Colors! Lite a go, too. It was pretty good, and judging from the small amount of messing about I did, a lot more versatile that Sketches. But Brushes is the best I’ve used so far. It’s more expensive than the Colors! Lite (free), and Sketches (£1.19), but it’s still only £1.79.