Using an RGB LED and a CDS photocell, I created a color sensor for use in a robotics competition. The sensor holds both the LED and the phtocell and provides a baffle between them so the CDS cell only sees the reflected light. By observing the amount of red, green and blue reflected, you can determine the color.

Videos:
Part 1: youtube.com/watch?v=wpiCV7Leegk
Part 2: youtube.com/watch?v=ccXk3cwn0Bk
Part 3: youtube.com/watch?v=sgW6iJvywh4
Part 4: youtube.com/watch?v=jCdaVU2rXHg
Part 5: youtube.com/watch?v=mTKlPUFRejY