Z Bar Paint Color

The Z bar that I have came from a '74, so it's possible that they started zinc plating them later on for the same reason they painted replacements.

Edit:

For what it's worth, the Rust-Oleum Stainless Steel color paint duplicates mild steel color pretty well. Wherever we have a car that we're restoring that gets overspray on bare steel or something like that, we use that first, before overspraying the color that goes on over the bare steel and it holds up on low temp stuff.