I have some rainbow LEDs from user hktaiyuen on Ebay. (These, specifically.) They change color from red, green, blue, yellow, cyan, magenta, white, and then repeat very fluently. Unfortunately if you have multiple connected up to one source then they will become off-sync, but it still looks very good. Here's a demonstration of these LEDs in use.
[media width=200]http://www.youtube.com/watch?v=UhY_xIFd4sY&feature=related[/media]