Well the tank portion that is discolored is metal, where as the other portion of the tank is plastic. I would make a warranty claim on that.
I'm fairly certain its the OEM paint as the bike was never titled, was sitting on the floor for two years. Dealer never mentioned having to repaint it from shipping damage or the like.I believe the clearcoat is yellowing cause of uv light. Are you sure it's the oem paint?