First select all the parts of the car that must be affected by your work. I have chosen to only modify the body of the car, without the windows, wheels and front. Select all of them using the polygonal tool. If you can’t select all of it in one piece, use Shift to add more to the current selection and Alt to extract from it.

Now, that you have selected all of it (do not forget to select out the lights, the mirrors and any decoration the car might have), Copy and Paste it. You can do this from the Edit menu or using Control-C and then Control-V.

Next, Desaturate the layer by using Image/Adjustments/Desaturate (or hit Shift-Control-U).

Next step, create the comics/cartoon-like color of the car. Since white is white no matter what, I chose to use a darker black color, to make the effect more obvious. Go to Images/Adjustments/Curves and choose something similar to what I’ve chosen.