There are many different methods of doing this, but i have found this the most effective.

1. Carefully select around the part you wish to colour change.

2. Copy and paste the selection to create a new layer. (or press Ctrl + J)
http://img182.imageshack.us/img182/1793/part1mh3.jpg
Desaturate the layer (Ctrl + Shift + U)

3. Go to Image > Adjustments > Color Balance.
http://img103.imageshack.us/img103/4449/part2zb9.jpg
Turn all settings up to 100 on the color that you want, on midtones, shadows and highlights.

4. Set blending mode in the layers box to Hue.
http://img239.imageshack.us/img239/1992/part3br6.jpg
you could use color, but hue has a better finish.

Here is the final result.
http://img103.imageshack.us/img103/3069/finalug0.jpg

Any questions? i will try my best to answer them.

-Nicecar-

Ay go to the tutorials part and check out my one on Lambo doors, and tell me what you think.

Alright, whatcha think of my 2, and yours is a lil rusty, but gets the general message across