Pick is a simple open-source color picker application for Ubuntu Linux. It supports history by remembering last colors where you got them from.
Pick lets you pick colours from anywhere on your screen. Choose the colour you want and Pick remembers it, names it, and shows you a screenshot so you can remember where you got it from.
Zoom all the way in to pixels to pick just the right one. Show your colours in your choice of format: rgba() or hex, CSS or Gdk or Qt, whichever you prefer. Copy to the clipboard ready for pasting into code or graphics apps.
For Ubuntu 18.04 and higher, you can easily install the tool from Ubuntu Software as it has been made as snap package.
For Ubuntu 16.04, open terminal (Ctrl+Alt+T) and install
sudo apt-get install snapd
Then install the Pick color picker via command:
sudo snap install pick-colour-picker
Once installed, launch the tool from application menu and enjoy.
I was using Gpick, but this is way better.