Sitting in the left corner of your status bar, until you are ready for it to jump into action Palette Grabber for Firefox, will grab the color scheme for any site on the web you are viewing.
Here is a little more about the extension from the developer:
Palette Grabber is a Firefox extension for web designers. It adds an “Export color palette” command to your Tools menu; selecting this command will extract a color palette for the current page and export it to a Photoshop/Illustrator color palette file. It’s useful for creating mockups of new designs which share a color palette with an existing design.
The Palette Grabber extension supports Photoshop, Flash, PaintShop Pro, GIMP, Adobe Fireworks, and Mac OS X Color Picker palette formats. You can pick it up from the official web site at http://www.gusprevas.info/palettegrabber/.