Install gpick by entering the following commands in the terminal:
sudo apt update sudo apt install gpick
Description:
advanced GTK+ color picker
gpick is an advanced color picker used to pick colors from anywhere on the screen, mix them to get new colors, generate shades and tints, and export palettes to common file formats or simply copy them to the clipboard.
Homepage: http://www.gpick.org
Version: 0.2.5+git20161221-1build1
Section: universe/graphics