The correct color is gunmetal gray. Drums were left bare and unprotected when new. Depending on how bad the rust is, I would put a cup brush on an angle grinder and brush heavy scale off, then coat with dark metallic grey manifold paint. If the drums are almost new I would sandblast them and either apply the manifold paint or urethane flat clear depending on how much I drive the car. The flat clear will yellow with heat but looks closest to factory fresh and keeps the drums looking great. But I generally only worry about the drums if the rest of the car is mint. Black is a functionally better color for the drum to be as mnecaise said, but if the drum is visible it will look incorrect. Personally I would paint it black if it was a stock show car, black if it was a hot rod. Wait, if it was a hotrod it would have four wheel discs
