That is a patch of some kind, possibly thin metal, or perhaps wood. These types of patches show up all the time on double sheathed NP and GN boxcars in the later 1950s and 1960s. I think they are a simple RIP track expedient during an era when milled car siding might not always be available, especially on roads that might not have any DS boxcars on their rosters.
Some I have seen are thin sheet metal, others are plywood or boards simply nailed to the car side.
I believe the light color is a coincidence, and not in any way an indicator of special service.