Just an optical illusion, caused by the position of the lighting. The car looks darker, in less light (or perhaps different colored lighting). If you look at the scene where Rossi is getting in, the door is open and it looks lighter, there there is more light than the back of the car. The car, a 1947 Pontiac Torpedo Sedan, was referenced in an earlier episode (Alchemy) as being vandalized. Obviously, it been restored since then.