What year? What diagram are you looking at? If we are talking 69-78 SOHC CB750 then green is ground, black is hot. Some wires, like the wires from the turn signals, were all black with a little colored ring at the end, such as orange, light blue, and the turn signal ground (usually just the rear) could be black but would have a little green plastic ring at the end where it connected to the harness.