If you want to know color value of any area of any webpage you need a color picker tool. There is a free tool for Firefox which can pick color value on any page.

ColorZilla is Firefox add-on with ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes.

Download ColorZilla