It is known that nice color picker application
gcolor2 ("Simple GTK2 color selector and picker") was removed from repositories and does not exist in Ubuntu 18.04 LTS.
But we still can install it by manual downloading from Ubuntu 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 use it as before:
Hope this helps
Note: post was inspired by this question on AskUbuntu.