What do these letters mean on this headlight switch

"The color of wire?" Well you have to trace it in the diagram and see where it goes. For example, yellow when associated with that switch is ALWAYS the ground circuit for the dome lights. Those wire numbers are IMPORTANT. They carry through the entire circuit. EG look at the dimmer switch and jot down the wire numbers. You will find one of them goes to the light switch. If you look at the engine bay headlight harness, the hi and lo beam connections go back to the dimmer switch, and the wire numbers/ colors are the same