The quick guide:
Select the part of the car that you want a different color (The lower half for instance). copy that selection, and paste it as a new layer. You can then adjust the color, swap colors on just that layer. To select JUST that area of the layer, press the control key down, and click on the thumbnail of the layer in the layers palette.
PhotoShop is complex as you know, and a step by step would take a LOT of explaining, but that is the quick and easy way to get started at least.
[This message has been edited by JohnWPB (edited 12-17-2010).]