Do you want to know how it's done? Learn how to use Photoshop the easy way!
A buddy of mine recently got a new motorcycle. It was painted purple with some lighter purple flames. I took a picture of it and then proceeded to change it to green. He was amazed and wanted to know how I did it. Here is the tutorial.
I started with this picture.
The first thing you will want to do is select: Image/Adjustments/Replace Color from the menu bar. Now you should have the "Replace Color" window open, and when you move your mouse over the original picture, it should show the eye dropper tool. The eye dropper tool is for selecting colors. I am going to click somewhere on the purple part of the bike.
At this point, you will need to adjust the "fuzziness" until the black and white image in the window seems to cover the area that you want to change colors. You may have to play with this a bit one way or the other. Also try selecting a different area with the eye dropper if you can't seem to get it quite right.
Now all you have to do is play with the Hue, Saturation, and Lightness sliders in the same window until you find what you are looking for. You can see the results in real time as you are changing them, so that makes it even easier. I am going to move the Hue slider to -135. I end up with a shade of green. Here is a before and after.