you dont need layers for a paint job, press on the lasoo tool and hold it the press the pologonal lasoo and select around the whole car, hold alt and deselct windows, lights, blinkers etc., Image>adjustments>desaturate, image>adjustments>variations, then work from there.
heres a tutorial webstite that should help you.
http://www.digimods.co.uk/tutorials/tutorials.htm