I have the Ultron Piece. It's the same effect on the Doctor Doom piece's too. You're comparing seeing black wear on a dark metal color which is bound to look very similar to white cloth with very visible yellow stains.
Ryu is weathering on Cloth doing stains. The staining spreads out a little on cloth unlike paint on a solid surface.
I have the Battle Damaged Ryu as does my neighbor. It has stains and a texture of dirt and debris added to him on top of that. Mine had much more black debris than his. Also the black debris had a real debris type effect that if you rub it hard enough, it can come off making it look more different than another same piece. The stains also spread in different patterns. The painters try to hit the same spots but being yellow on white it makes it more visible and noticeable than black on dark metal gray.
