They were zinc plated plated.
Consider the difficulty of painting a chain. Everywhere two links touched wouldn't be painted. Getting paint evenly on all sides would be extremely challenging. Paint would immediately start chipping off under normal use. Would GM waste the money to do that in 1950? It was just off-the-shelf, steel chain, cut to length and installed.
I've never seen painted chains or or remnants of paint on chains of unrestored trucks - even stepside Chevys all the way into the 1970s
