A Xournal++ plugin which maps keypads keys to (default) colors
First clone this repository to your plugins directory:
git clone https://github.com/gapato/xournalpp_keypad2colors ~/.config/xournalpp/plugins/Keypad2Colors
Restart Xournal++, you should be able to activate the plugin under Plugin > Plugin Manager > Keypad2Colors.
Restart again to load the plugin.