From the official website:
"Colors! is a simplistic digital painting application for Nintendo DS based on modern
painting-techniques developed for drawing tablets in programs like Photoshop. By
taking advantage of the pressure sensitivity of the DS touch-screen it becomes a
perfect portable digital sketch-book."
This is what homebrew is all about, and frankly it's the coolest thing I've ever seen on the DS. For those of you who have never heard of "homebrew" or thought the DS was only capable of playing games, check out the Colors! official website. There is a FAQ explaining what this homebrew stuff is all about and a gallery where you can see more really impressive paintings created with a DS and this brilliant bit of homebrewed software.
For another brilliant example of homebrew check out this video of a kid controlling his computer with his DS.