Some are red oxide primer. That is what the bodies all had on them. Most were primered gray over that but I have seen some that were not. The gray was never that thick and usually was the cause of the paint flaking off. When ever you would see a car with the paint coming off it was usually down to the red oxide
That makes sense actually my car had C& and a repaint to silver in the 80s so i went through a few layers but I vaguely remember the red right before the metal.