Ok, some explanation:
First I created lots of paths for all parts/sections of the car. With a little training working with paths can be quite fast, and results in well defined and smooth selections.
It’s important to recreate every curve only once. For example, using one path follwing only the inside of the left lights/glass. An intersection with the main outline results in the final selection.
I work with the airbrush and a custom brush, with the editor window always open, often changing size and hardness.
Now, on the bottom you see the base color layer, in combination with all window, interior and lights layers plus highlights (white). Base color on seperate layer allows to change the color, without rebuilding the shading (even though some adjustments might be needed for drastic changes).
Above reflections from ground, created by using the color picker (use Ctrl to switch to color picker, from any drawing tool).
Reflections from the sky.
Light (on black ground just to make it clearly visible).
Hope this is of use, don’t hesitate to ask questions or post comments on this technique.