Note: Product discontinued by manufacturer, page for reference only. The ADJD-S311-CR999 is a great little color light sensor. In fact, it's so little that it can be pretty difficult to play with, so we made this evaluation board! It provides all the necessary support circuitry to discern the smallest differences between visible colors.
The Parallax ColorPAL combines an RGB LED, a light sensor, and a microcontroller to make a color sensor that can also be used as an ambient light detector and a color generator. Readings are reported via a 1-wire asynchronous serial interface.