How should I prep for a color change over existing good paint?

Just curious as to what color the factory primer/sealer under the red on her car was? I had to get the tailgate fixed on my red Dodge truck. The shop used light gray primer/sealer and sprayed the red and clear, which turned out beautiful except being the wrong color. Factory put black or dark gray under the red. Now that I know more what I'm doing, I'm gonna redo the tailgate and a nice factory step bumper so it will match.