The AKAI APC20 is a MIDI controller, which has controls that light up on it. The lights correspond to the status of a clip in Ableton, but is there any way to change these lights by issuing MIDI commands to the controller using another device (ie. my laptop)?
Absolutely. That is how it works. See http://www.cycling74.com/forums/topic.php?id=20355#post-106507 for basic mapping information. Basically, different velocities are for different LED colors.