This is just an example of how to quickly change the colour of a car with CS5.
Open up the base image.
Zoom in so you can see the pixels.
Using the polygonnal lasso, select the areas that will be painted another colour. (Wing mirrors, Grill, Wheels, Lights) Then cut these using the cut tool, and paste them into additional layers.
Select the base car image again in your layers.
Open Hue and Saturation from this menu.
With values set as default, your car will look standard, Move saturation up and down to change the colour around slightly.
If you move saturation down, the colour will drain out (Less colour, more greyscale) If you move it up, will be more vivid.
Then open up Brightness and Contrast from the same menu.
Turn down the brightness for a darker car, turn up the contrast more more shadowing basically...
This is literally a 2 minute guide... You can spend a lot longer to make it look better!
As for the widemouth and colouring different parts. Just go to your new layers, mask off again with the lasso and just pencil in the new colour with the pencil tool.