The color matches the exterior body paint color. And as Challenger RTA suggests, the amount of paint varies. The production line worker is to spray the exterior, and the underside of the deck lid, and the area surrounding the opening where the weather stripping goes. Perhaps even some or all of the hinge mechanism. That is their focus. In the process, other areas of the trunk get sprayed "by accident" or, sprayed with less emphasis on getting a complete and nice looking coat.
Some concourse restorations attempt to mimic this lazy spraying of paint. Others do not. Originally, there was some randomness to this at the factory. But it's common knowledge the trunk areas are prone to rust, perhaps a result of the lazy painting. It's hard to say. And during a restoration, when people are going to all that trouble and expense, perhaps even replacing the trunk floor pan in the process, it's pretty common to give this area some extra paint and attention.