I've been helping a friend with her media arts artwork this year. The idea was that each musical note could be associated with a colour. For her artwork, she hung 48 lanterns around a room and had a midi keyboard on display. As you started to play the keyboard, each light (4 light per note) would light up. There were 3 ultra bright LED's and a resistor wrapped in different shades of cellophane to produce the different colours. Playing different combinations of notes would allow different light combinations, the harder you hit the notes, the brighter the colours. The colours would fade with the sound.
You can see a blog post about it here: http://www.scottyob.com/?p=4