The only reflector I ever had to paint was the '55 Ford round front parking light reflectors. They were factory painted with the same Argent Silver that is used on the front splash pan and oil-bath air cleaner housing. I just duplicated what they did.
My guess would be that the bulb would reflect a little better if they had been painted white, but who cares. It's not shining a path in the foggy darkness. It's only function is to let oncoming drivers know they are there.
On '50's Fords, the taillight housings are chrome-plated, so that takes care of that, and these have sealed beam headlights where the reflector is an integral part of the lamp.