1. Get a good car picture like the one I've chosen.

2. Now use the pen tool, the lasso tool or the magic wand to select the body part of the car (the red part in this case).

3. Create a new layer above the car's one and fill the selection with a color of your choice, I chose a blue shade.

4. Looks not good till now ain't it? you feel it is not realistic ! don't you ? That's why we're going set the blending mode of the layer filled with blue to 'color' mode. You can add more details if needed.

Here are More examples of what you can do with this technique ;)

5. Change the color with Hue/Saturation

6. Make a selection with the polygonal lasso and deleted the selected part of the layer filled with blue.

7. Adding some details and color thanks to the pen tool

8. Filled the selection (step 3) with the spectrum gradient.

That's all !! Enjoy coloring the car for now :)

P.S: If you want a ride just tell me ;)