I recommend GPick:
sudo apt-get install gpick
Applications -> Graphics -> GPick
It has many more features than gcolor2 but is still extremely simple to use: click on one of the hex swatches, move your mouse around the screen over the colours you want to pick, then press the Space bar to add to your swatch list.
If that doesn't work, another way is to click-and-drag from the centre of the hexagon and release your mouse over the pixel that you want to sample. Then immediately hit Space to copy that color into the next swatch in rotation.
It also has a traditional colour picker (like gcolor2) in the bottom right-hand corner of the window to allow you to pick individual colours with magnification.
