Back in the vacuum tube era there was a RETMA standard color code for internal wiring of electronic gear:
Black - Grounds, grounded elements and returns
Brown - Heaters or filaments, off ground
Red - Power Supply B-plus
Orange - Screen grids
Yellow - Cathodes
Green - Control Grids
Blue - Plates
Violet - not used
Gray - AC power lines
White - Above or below ground returns, AVC, etc.