*snickers* Why would you need 16 million colors...especially if it's pixel art, or "spriting"? It's fine for a photoshop alternative, but not for spriting.
Anywho, you can use quite a few programs... something as basic as MS Paint, something in the middle like Graphics Gale or Paint.net or, if you have the money... Cosmigo ProMotion was designed specifically for pixel art. But I wouldn't reccomend it, especially to an 11 year old beginner. Just try Graphics Gale out first.