First select a car you wish to paint.

Select all the body work of the car. I used the magnetic lasso toll to get the rough out line. Using Alt+click sets it to subdract so then went round the lights ect.

Got the free form lasso tool. using combo of shift+click and alt+click to get a nice selection around my car.

Duplicate this to its layer.

Now you will want to re duplicate you body work layer, and keep the first one incase you make a mistake. Place the one original body work layer below your car (so that you cant see it) then place the 2nd above you car.
For one shade of colour just hit ctrl+u and adjust your slides and watch as you car changes colour.

You can take the effect further and make pearlescent by doing the following.
Duplicate your body work layer about 3 times (more if you wish more colour's in you flip paint/pearlescent).
Take out your eraser tool on large soft brush, set the opacity and fill to about 35-40% will do ad just rub away to on you top layer to let the next layer show though.

It doesn't need to be great yet as we neaten it up later. Repeat on the next colour's layer.

When you are done merge your coloured layers together and then duplicate the paint layer. And try out the blending modes, multiply and vivid are good ones to finish off your effect, but depending your colour's you might need a different setting. for example.
Using multiply with the colour's i had made the image darker

But vivid light enhances effect.

With some practice you can create a really nice finish.