The discoloration is from a combination of sweat, skin oil and hair product. That's the danger with brown/tan leather jackets (I'm assuming that's what you have). I don't think there's a sure-fire fix, but I would try repeated applications of some kind of foaming leather cleaner like Lexol. Perhaps you can eventually draw out the dirt/oil. Worst case, you have to recondition the collar.
10/10/15 at 8:37am