There is no way to keep python3 as default on Ubuntu 16.04, because many programs including apt are written in python2.
And there is absolutely no need to set python3 as default as you can run python3 apps using python3 appname or setting python3 in shebang.
You can set back default to python2 by running:
sudo rm /usr/bin/python
sudo ln -s /usr/bin/...
Is the Multiverse repository already enabled? Check in Software and Updates (the fourth line of repositories). If not, do sudo add-apt-repository multiverse && sudo apt update or enable by clicking on it then clicking [Close].
Once Multiverse is enabled, run sudo apt install flashplugin-installer then close Firefox if it was open.
Relaunch Firefox ...
One can get an idea of the software offered by Ubuntu Mate's Software Boutique by downloading the contents of this link as sb.html and then running
grep -n name sb.html
The list has 5782 entries currently. gcolor2 isn't one of them.
However, in gcolor2 package missing in Ubuntu 18.04?, there's an answer describing how gcolor2 can be installed in any 18....