[youtube=http://www.youtube.com/watch?v=DqboDjQX0UE]

[Kyri] has made a simple circuit to do color detection using an RGB LED. Simply set the LED to the color you want to detect then wave the object over it. Another LED will light up if the object is the same color as the LED. The detection is done by a photo resistor. The theory is that an object will reflect more of the light that matches its surface color. She shows that this kind of detection could be useful for sorting in robotics.