use tha lasso tool. i personally use the polygonal lasso tool, dont worry that it makes lines, it will adjust itself to the body. you might wanna go a tiny bit wider then the body, cos after that you can erase the bits that are wide rather then have a too tight selection and have still some of the old paint.
So do the following:
1, take a lasso tool
2, select the car body (make it a bit wider)
3, if your selection is messed up, you can add to it by hitting SHIFT + Lasso tool or take away from it by ALT + lasso tool
4, make a new layer by right clicking on your selection and hitting Layer Via copy
5, play with hue/saturation/brightness etc till you get your desired color
6, take the Erase tool (E) and erase parts of the paint from those points where its too wide
sounds a lot, but its 5min max