Actually, i have another arduino sending the colors.
In order to generate the colors, i used 3 TLC5940, 16*3 homemade vactrols, and 16*3 low-pass filters. The TLC can generate 16 PWM waves, running into LEDs, facing LDRs, going thru a LPF, part of a voltage divider, running into an opamp. I would certainly simplify this if i had to make it again, but i was running out of time and used only things that i knew they were working.
You have to know that the color signals must occur only when you're supposed to see colors on the screen. I've spent days to figure that ;D, just because I thought that the screen was "reading" color values when it needed. It means that the second arduino (color manager) has to be aware of when it have to send colors, and when not. Luckily, i found a video-purposed opamp (AD813ANZ) that makes it easy to do, because it has three "disable" pins. These pins are plugged to the pin of the first arduino (sync manager) that is LOW when color can be sent, and HIGH otherwise. That's the purpose of this special pin.
Concerning the ground cables, i soldered them all together, as i was using the same ground reference. You have to do some experiments, as i had sometimes to deal with noise. I used CAT5 cable (8 wires, cheap and reliable), so i had not that much possibilities for wiring. I had to make 16 cables, 6.5m each, worked well. Using the same ground or not depends on if you use the same ground ref for sync and colors. If i remember, each wire (2 syncs and 3 colors) has its own ground wire, so it means that you could send 5 signals with 5 differents ground refs : the screen would accept, as it's isolated (that's the assumption i made ;D )
Picture of the beast :