1. Open the picture that you want to use. I'm going to use this car for this tutorial.

2. Take out your "Magnetic Lasso Tool" and mark around the whole car.

3. Now there might be some parts that you don't want to change the color on. For example the small background spots on mine. So just use the Magnetic Lasso Tool again, hold the Alt button and select those parts you don't want to have.

4. Click Ctrl and U to enter the Hue/Saturation menu. Now just mess around with the settings (do not select Colorize) and find the color you like. Or change the saturation to and change the color by clicking Ctrl and B (Color BAlance).

5. Now deselect the markings (Ctrl and D) and you can see the color much better. If you don't like it. Change it again.

Now you're done! Here i s my results: