ColorZilla is a Firefox plug-in that enables you to determine which color an element on a web page is using. Click on the Eyedropper tool in the bottom-left corner of your Firefox window. The cursor will turn into a cross and the html element you’re hovering on will have a red border. The color values of the element you click on will be added to the status bar.
ColorZilla also contains a color- and palette-manager. You can open it by right-clicking anywhere on the page, pointing to ColorZilla and selecting either Color Picker or Palette Browser.