Being a '76, that's very likely to be from some sort of smog control device that is no longer intact. But wait for confirmation from someone that is more familiar with those years. Maybe Viperwolf has a schematic, or jackkys or blubronco can check theirs out (they've both got '77's I think, but might be the same in this case).
Might have been just the carb anti-dieseling solenoid if your year had that, but mine was like a Black w/blue or just Blue or something like that if I remember.
I'll check my diagrams anyway, just to see if it's there.
In the meantime search out Steve83's posts here (using the advanced search function, or just look for something he's posted in the last day or so) and when you find one click on his username and hunt down his SuperMotor's web pages with the re-worked wiring diagrams in living color (and soon to be 3D even!%)
and see if you can find that color combo.
Good luck.
Paul