You have to "really" know how to use PS to color cars, if I said you need to use the alpha channels you would have know idea what I'm talking about. Channel masks let you keep the the highlights, shadows and midtones while putting on any skin you wish. I could make you whole car tiger print if I wanted to.
gen7fan is right about this, but i would never try to explain channels and everything about mid tones, highlights and shadows, its too complicated to explain and this isnt really a photoshop website, i try keepin it basic for members
if you really want to learn, buy a photoshop book, or look for online walk thru's