color organ

Neither a musical instrument nor a body part, actually a rudimentary lighting controller that flashes lights to sound frequencies. Most have three output circuits, corresponding to bass, midrange, and treble frequencies.

See the video for one example:
[media]http://www.youtube.com/watch?v=l3IMeA6ZdKY[/media]

Once a staple of every "learn electronics at home" kit, primarily by Heathkit, similar units are still being sold today:

http://www.amazon.com/Velleman-Channel-Color-Organ-Kit/dp/B0002Z82Y4