If you have photoshop or MS paint (photoshop is better because you can do all the editing on another layer), find a base model of your car in this cartoon front back side thing, and start the pixel by pixel editing. To change a car color with these, assign each shade of the default color with a shade of color to your car. For example to make a red car blue, all dark red values should be given a corresponding dark blue value, and all pale pinkish values should be given a sky blue color. Whites stay white, blacks stay black, get creative for aftermarket parts. That's all I can tell you is manual editing. Pain in the a$$ but it's the only way.