“Color Demo”is a demo by Peebrain that shows the effects of Color on a GBA:

Whipped up this demo to look at the color capabilities of the GBA in a nice pseudo-GUI environment. It has three”sub”-demos. Use L and R to cycle through the demos. The source is included as well. I tend to write a lot of Qbasic code to do some tedious work, so I put those in there as well. Intended to be run on the hardware, not emu’s (although it works fine in them).

http://gbadev.org/