Here is a quick way that I use to change the color of a car, it is reasonably simple, and allows you to change to almost any color combination.
First of all, open the picture you want to change color, create a layer to work with then use the Polygon laso tool, and zoom right in on your image so you can get reasonable detail when selecting the outline of the car.
Use the tool to select the whole car. then fill that layer with the color you wish to use.
With the color layer selected, use different blending modes to get different effects and colors, for the one below, I used the Color Blending mode.
Once you are happy with the color, use the eraser tool to erase the color from the parts that arent part of the duco, such as windows, decals, lights etc..
And thats about it! If you save this with the layers (as a .psd or similar) you can come back, and simply refil the color layer with a different color and always be able to change the color on the car no worries.
Once you work out the best way it works for you, you can use the same concept to tint windows, lights, wheels etc..