i use to work on these on the side some the wires are the same color that i have always seen both white both black usually. it don't matter it is an electro magnet. one hot (+) one neg. (-) and i never liked grounding directly to the frame run two wires back there for the brakes and ground to trailer frame also. and besure you run the ground wire to the trailer plug so they can get the ground through the plug not the ball because it don't always ground good. the ground can give you alot of headaches
if you use the trailer frame only and go through the ball when you go down the road empty your running lights and such will more than likely flicker when you hit bumps so be sure to use the plug.