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.
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 
@Norbert_X The PPA doesn't exists anymore. Can you update the link?
I have e-mailed PPA owner about the packages in his ppa:evertiro/gcolor3 PPA.
By the way there is no such gocolor3 package in official Ubuntu repositories.
In PPAs this package is still available for Ubuntu 16.04 LTS at ppa:varlesh-l/ubuntu-tools.
But repology.org says that this package is available for example in ArchLinux. And they mark https://www.hjdskes.nl/projects/gcolor3/ as official developer's page. It provides FlatPak, so you have to use it with:
flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
flatpak install flathub nl.hjdskes.gcolor3
# run
flatpak run nl.hjdskes.gcolor3
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