It depends on the year and application.
All disc brake master cylinders from '67 to '70 were painted semi gloss black as an assembly; cap, body, bail wire.
The bodies of '67-70 drum brake master cylinders were e-coated black prior to machining, meaning the machined surfaces such as the outlet bosses, piston retainer boss, flange and cap mating surface were bare metal. The caps were plated a thin gold color and the hold down bolt and bracket were dull gray.
From '71 forward for at least a few years both drum and disc were coated a sort of dull, gray phosphate, cap and all. The exception was the '71 hemi disc brake mc which was a carryover from '70 and painted black.