I just found this Ace hardware primer, red oxide, and I loves it. Ace primer goes on thin, so it's nice for preserving details. But... I have no paints like it that I can match up with!
It's a brownish red (and oddly enough, google returns no photos of it alone...)
Does Games Workshop make a red oxide? I don't need a perfect match, as it'll likely get covered with wash and other colors... But I don't have anything even close
It's the one in the middle