gcolor2 package missing in Ubuntu 18.04?
It is not too difficult to install it manually (from 16.04 LTS xenial repository):
cd ~/Downloads
wget http://mirrors.kernel.org/ubuntu/pool/universe/g/gcolor2/gcolor2_0.4-2.1ubuntu1_amd64.deb
sudo apt-get install ./gcolor2_0.4-2.1ubuntu1_amd64.deb
And then use it as before:
Notes:
1. modern alternative - gcolor3 is packaged in very few distributions.
(see at repology.org).
2. on Ubuntu gcolor3 is installable from ppa:evertiro/gcolor3.
3. currently gcolor3 does not have broad list of predefined colors (see above, reported bug 110 to developer).
4. another alternative is KColorChooser, it is installable from repository as kcolorchooser package, but it does not have list of named colors.
zenity --color-selection provides a no-frills color-dialog. In 19.04, but not in 18.04, the "dropper" in the top-left corner allows you to click on any item on your desktop.:
There's also yad which is said to offer more features than zenity. And the version in 18.04 comes with a dropper to allow you to pick a color from any item on the desktop:
yad --color
If you would care about a gcolor2 alternative, I am recommending Gpick.
It's easy to use and has a lot of features, which you can find in Gpick's website. You can install it by running:
sudo apt install gpick