Garr Reynolds over at Presentation Zen kicked out a cool review of a web tool called Kuler which you can use to create a color theme for your presentation. If you have a photo at the core of your presentation, you can extract colors from it to create a palate for the other elements in your slide. As you can see in the picture, it makes for a nice effect.
I can think of a few times when I had to fiddle with the colors of a slideshow and this would have been nice. I think this would be more useful if you are doing a series that has one, central image, or if you have some branded artwork that you want to integrate into your presentations.