If you are working on web design or UI, you might sometimes be curious to know the RGB values of colors used in GUIs or web pages. A color selector tool like gColor2 makes this job effortless.
It can be installed easily:
$ sudo apt-get install gcolor2
Run it and it shows a dialog with a color wheel, a pipette and RGB values. Click on the pipette to select it and use it to click anywhere on your display. The color picked by it is displayed on the color wheel and its RGB values are shown accordingly.
Tried with: gColor2 0.4 and Ubuntu 12.04