Modification of the fontcolor plugin allowing the Moodle admin to define colors.
You can have several rows of colors, not just one single column : in the plugin settings, just enter several space-separated hex color codes per line -- thanks to Abu Zakaria!
If allowed in the plugin settings, users can now use a color wheel to set their own colors -- thanks to jinhofer
Install the plugin directory in lib/editor/atto/plugins.
In Site Administration > Plugins > Atto > More font colors, you can define the colors you want your users to be able to chose from and other options.
To make the button available in the Atto HTML editor, enable the plugin by adding morefontcolors in the Atto toolbar settings.