Light metal parts are not painted with a few exceptions. I've seen factory photos with painted transmission covers, but not sure if I've ever seen evidence of paint on covers. Steel castings are painted. Most standard hardware is black (black zinc phosphate is a good choice over black oxide). Vehicle specific hardware is iron phosphate, the parts that look grey/brown. Not all platers do this now. There is no zinc chromate (gold) in these cars. There is nothing wrong with plating parts with black zinc phosphate prior to painting, for extra protection, especially if the part has some exposed threads, or a rubbing surface where paint may rub through. I'm thinking suspension parts, hood and trunk stays, various brackets etc that were painted.
The silver colors are either cadmium or zinc, sometimes hard to tell which over time. Outside supplied parts from Bosch and Solex may vary from the black hardware rule, Bosch small hardware can be cadmium plated, like the coil strap and terminal screws. Carb and fuel pump hardware is almost all bright, probably cadmium originally. The air cleaner latch hooks I think are black and the wire clips are bright. Practically speaking, given the cost of cadmium as well as minimum charges for each process, it's hard to justify doing cad on the bright hardware except on the most fussy restorations. The zinc looks fine.
As for painted parts, (trans case, motor mounts, starter, generator, ignition wire tube, gear shift bearing body) semi-gloss is appropriate although I do engine blocks in gloss as I think it holds up better. Side covers are painted also. On the wiper motors, we are using a grey/silver paint on the steel, and leaving the zinc parts natural.
