Norm, I am usually guilty of never using 10 words to answer a question when I can get away with 50
The short answer to your question is that you do not need a high-end piece of software, in fact the picture editor that is included with virtually all Windows OS, called MS Paint, is really pretty good for a freebie. I use it a lot for marking up construction site inspection photos etc.
You certainly don't have all the whistles and bells of an Adobe photoshop, but the airbrush tool in paint would let you "repaint" the entire car...in about 3 coats....and there are other tools that would allow touching up just the bolt heads.
You can customise/define colours as well.
Rgds,
Tim