Not sure which "Mast Light" you're referring to. There are generally 3 possibilities:
1. Steaming Light: White light located halfway up the mast, illuminates +-120 degrees forward. Often labeled as "Bow Light" in the '70s. Required when motoring at night.
2. Anchor Light: White light located at the masthead with 360 degree 2-mile visibility. Often labeled "Mast Light" in the '70s. Required when anchored at night.
3 Spreader Lights: White downward-facing light(s) for illuminating the deck. Optional.
In my experience, there was no consistent color coding used by Pearson in the '70's. My own P-323 was wired up with ordinary THHN house wire, which is totally inappropriate for a marine environment. And as a result, everything is White, Black, or Green, regardless of function!
According to the Marine color coding for wires (
http://www.marinco.com/en/resources/wire-data/marine-color-code), as published by Marinco, the wire from the fuse or switch to any navigation lights should be Gray.