All Midland/Ross style boosters(the ones with a band holding the two halves together) should be plated in zinc dichromate. That is the rainbow/gold finish...the same finish used on the master cylinder covers.
Do you have power disc brakes or drums on your 72? I believe the drum brake boosters are painted black.
I sent my booster to Booster Dewey for a rebuild/replate and it turned out really nice.
