I have a professional version of Photoshop so I hope mine is the same as yours:
Select the body of the car by drawing a path, or using the lasso tool. Select the color you want in the picker palette and make sure it's the foreground swatch in your tool bar. Now go under the edit menu and select fill. Make sure it says "color" in the pull-down menu where it normally says "normal." Click ok. Now you should have colored the body of the car and kept all the highlights, midtones and shadows.
First you want to select the colored areas of the car that your will be color-shifting. Make sure you dont have wheels, lights, glass,etc in your selection. You can make an accurate selection in several ways;
You can use-- a quick mask, the lasso tool, the magic wand, magnetic lasso, extract tool or any combination of them. Personally I prefer to use the path tool and draw my selection with bezier lines, then load the path as selection. Take your time and zoom in to about 300percent to get a good clean selection. After you selected just the parts you need to color shift save your selection.
With your selection active proceed to the Image menu
Image--->Adjust--->Hue and Saturation (apple+U)
Move the hue slider to change the color, the saturation slider to change the intensity of the color and the brightness slider to change the luminosity. Make sure the "preview" check box is checked. Also try the "colorize" check box in the lower right corner. Try many different positions with the sliders for different effects.
Another quick and easy way of doing it is to open your desired image.
Go to Image/Adjust/Replace Colour.
This will open a window with a smaller image inside it.
Choose the option 'Selection' under the image and use the eye dropper tool to select the colour you want to replace.
Any colours that match what you have chosen will appear whiter in the image.
Try clicking on the the bonnet and then moving the Hue, Saturation and Lightness bars to change the colour.
You will see the colour change in the original image.
You may need to play around with the fuzziness value to get it right.
Experiment, it can be very effective and is a quick way of doing it.