I tried to do this exact thing years ago too! Except I only used one RGB led, and thus could never sense the red channel, because the bandgap of green and blue LED dies is too big to really get any signal from red light. Isn't it annoying when your project is up against quantum mechanics?

I also build a swarm of throwies that used a single LED to synchronize their blinking. That project was much cooler, and I wish I had better documentation...