I have the perfect tutorial for you... and it works really really well.... just let me find it again.
edit: ok the tutorial is on a different hard drive that my stupid sister left at her friend's house.
I did a google search for "photoshop tutorial change car color" and heaps come up. But none are very good. Most just select the area you want to change the colour of by using a mask. Then fill with the desired colour and then change the blending mode.
This way doesn't look very good but it gets the job done.
I'll post up this other tutorial I have when I get it tomorrow... i can't remember all the steps off hand.