The different colors on trailer wires indicate the different circuit functions to which they correspond. For example, a standard four-way trailer connector has the colors white, brown, yellow and green that correspond to ground, tail/running lights, left turn/brake lights and right turn/brakes, respectively. Colors vary by trailer type and brand.
The colors and circuit functions of a seven-way trailer connector are identical to that of a standard four-way trailer connector but have three additional colors and circuit functions: blue for the brake controller, purple for reverse lights, reverse lockout for trailer brakes and auxiliary, and black for auxiliary 12v power.
Utility, stock and horse trailers typically use six-way connectors. These connectors use white, brown, yellow, red, green and blue for ground, tail lights, left turn, stop or electric brake, right turn and auxiliary, respectively. The six-way ASME color wiring code differs slightly, using brown for running lights and black for power. The RV color wiring code is identical to the ASME code, except for the functions of green and brown which are reversed.
Wiring codes differ widely among brands and models. Refer to the trailer's owners manual for an exact wiring diagram and color code. Always perform a test before connecting wiring.