There are many graphical editors out there which have been designed to offer the end user all kind of abilities regarding colorization or simply, image editing. From this category I can recommend a few which are most used by designers. By far the most powerful one used nowadays is Adobe Photoshop because of its powerful tools, features and support for different kind of plugins. It will basically let you release your imagination and paint the car of your dreams with the possibility to preview and then apply. Another one is GIMP, which comes in free flavor for everyday user and painter. It has strong points regarding art painting and nonetheless it represents a strong competition among other image editors. Although InkScape uses vectorial art it’s still a good choice for matte colors and image manipulation. As you can see the choice is yours to make regarding which one of these most important examples are good for you to use and preview your work, in this case, you car.