There really is no good existing standard for FIRST to copy. The NEC, UL 508, NFPA 79 all are mostly AC centered. With the NEC we can basically use anything but gray, green and white for our ungrounded "hot" AC conductors.
For FIRST I'd like to see something like red for +12, blue for +24, and yellow (or similar) for +5, and black for all negative conductors. Conductors in cables should be allowed to be re identified if you want. If you want to identify negatives per voltage twisting, tyrapping, lacing, labeling, taping (either "phase" taping the negatives or using tape to bundle) would be optional.
No matter what the color code is though everyone should get into the habit of using a meter to confirm. One of the first thing you learn in the real world is once maintenance guys get at it and contractors guys start applying WIT (What's In the Truck) all bets on color are off.
