In this tutorial, I am assuming that you have some basic knowledge of Photoshop.
Take a picture (such as this cute little Blue Aveo).
Now paste the exact same picture over it again, so that you have 2 identical layers (look on the right side).
Now using the lasso tool, select just the car more or less like so:
Now using Image -> Adjustments -> Hue/Saturation, play around with the settings until you get your desired color. (I chose funky green).
Now select the inverse (either from from the Select menu, or Shift-CTRL-I)
Then press the delete key. (This gets rid of the background off of that layer.)
Now click on the little eyeball next to Layer 1, that will make it invisible.
Now using the eraser tool, go around the car, and get rid of everything you can that isn't painted car (wheels, tires, lights, windows)
Then zoom in on the car, and using a really small eraser (size 1-3), go in and get all the stuff you missed. Pay attention to detail. Get in all the nooks and crannies.
Zoom out, and you picture should look like just the shell of the car: like this:
Now click on the eyeball next to Layer one again, and poof, your background comes back. And the car looks pretty.
That's pretty much all there is to it.