use the lasso tool, and zoom to like 400% to outline the part of the car that you want to change......
Then carefully choose your color and fill it in (push delete first to clear the whole selected area as to not have to fill in each individual pixel with color)....
I may recommend using a very gradual gradient so that the paint doesn't look fake....
To make this effect you can also (on two seperate layers) put a black fill in, and on the next layer up put your color fill and make the layer transparent (partially anyways)....
Once you fool around a bit, you can use tools such as chrome to really kinda dress it up...
Does any of this help or make sense? If not just let me know, I'll fill ya in with some more tricks if I can