Looks like someone stripped them right down to the bare metal, then dipped them in Alodine (conversion coating). Alodine turns the aluminum yellow/tan and is an excellent "primer" for aluminum to be painted. And since they were painted, that does makes sense. Only way I know of to get it off is sanding, polishing, or blasting. A very strong caustic solution might also take it off, but I doubt you want to mess with that kind of stuff.