Re: the current / power source, I was thinking of using a number of lithium 9 V batteries. Each battery should be able to power 40 LEDs for an hour or so (1,200mAh), which is enough for my needs. So 10 batteries should be able to power 400 LEDs. I don't need common wiring for all the LEDs (I could use more batteries, for example), except that they have to share the signal...
And by RGB signal I simply meant taking the 4 pins that drive my lone, single RGB LED right now.
Or am I thinking about this the wrong way?