Load up your car image and select the “pen tool”, draw around your car but not selecting your windows, grills, lights etc… so basically your just left with a shell. “copy and paste” to a new layer.
With your newly pasted layer selected goto “image > adjustments > desaturate” then goto “image > adjustments > variations” use the settings below.
To make a black car click “darker” about 2 times also marked in red on the image above. (to make your car even darker just click some more times) should have something like below.
As you can see it doesnt look very good so if we goto “image > adjustments > brightness/contrast” and adjust the lightness and contrast, the more you up your contrast the more shiny your paint becomes.
To make a white car open up your variations panel again but click original in the top right corner to reset your layer as it was, then click “lighter” you only need to click this once.
Now goto “image > adjustments > brighness/contrast” and increase the brighness to about +20, you shouldnt need to touch the contrast unless you want to make it more shineyer.