My old teammates Ralf & Steve, the guys who brought you Vanishing Point in Photoshop (and before that LiveMotion), have released their first iPhone application, ColorBlast. It’s a tool for selectively painting color onto images:
The core feature of ColorBlast is the ability to separate and color one object in an otherwise black and white photo. To make this possible, and even fun, ColorBlast employs selective color technology which allows you to start brushing on a specified color and then paint only that color.
Here’s the app in action: