A friend asked me how he could use Photoshop to pick colors for a new paint job for his house, and I believe that this technique might work well for that task.
In this tutorial, I was really quick and lazy, and decided to change the color of both of our shirts, since they both contained similar hues. If you only wanted to change the color of one shirt, that probably be more tedious work involving layers. You still would not have to exactly select a layer boundary that traces each shirt, but you might have to place each shirt on a different layer.
The impressive thing about this technique is:
1. It is very fast and easy to do
2. You achieve results that are still believable
3. You have a lot of power and fine-grain control in selecting hues to modify, and how you want to modify them.
Edit: This technique will not work well for all cases. It works best if what you want to change has distinct hues from other things in the picture. For example, if I wanted to change my shirt color, and it was redish/orange, it might be impossible to use this technique without altering my skin tones. More advanced, time consuming methods use complex selections, masks, and work with levels. This instructable only uses the hue/saturation dialog.