There is no code requiement, other than white or gray must be a grounded conductor & green or green with yellow stripes is a grounding conductor, and orange must be used for the conductor with the higher voltage to ground on certain systems.
There are, however, a few color code schemes that are popular. For 120/208 3 phase- black, red, blue. For 277/480 brown, orange, yellow.
These coloring schemes don't make the system work one bit better. They do make it easier on the installer & anyone that follows him. Nothing worse than opening up a box during a troubleshooting session & finding 20 or 30 conductors, all the same color.
