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:
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.
Or add the missing Opacity feature to MATE Color Selection (which is included).
BTW, if looking for a text editor, geany (in the Software Boutique), has a color picker built in.
There's Gcolor 3. Not sure though if the download link would in Ubuntu MATE as this is a mint post. https://mintguide.org/tools/381-gcolor3-121-nice-color-picker-for-linux-mint.html
I already know about gcolor3. We have discussed it on AskUbuntu as an alternative.
But we can install it directly from developer's PPA:
sudo add-apt-repository ppa:evertiro/gcolor3
sudo apt update
sudo apt-get install gcolor3
and we will get
Note: it misses list of named colors (compare with gcolor2 above). I have reported bug 110 on gcolor3 GitLab about this. So it looks as stupid as all modern GNOME
Neat. Gonna try it now.