There are several ways to paint cars... I always do my selections of all of the painted parts with the pen tool and then create a new layer and actually paint it like you'd paint a car.

Then that layer can be adjusted any way you want... or even have multiple layers.

There is no one right way to paint a car... some people just adjust the settings of the paint that's already on the car but if you do it that way be sure to have made a duplicate layer underneath the one you change so you can delete all of the non car paint parts like windows and wheels and BG.