Atto is the default Moodle editor. This is a 'feature' of the new html editor, Atto. It has been deliberately done. The html editor picks up the font family, colours, etc from the theme to maintain consistency throughout the site.
However, this does not necessarily suit everyone.
There is an option to add colours by using the fontcolor setting. To do this, go to <your site name>/admin/settings.php?section=editorsettingsatto. It is recommended to give this much thought before implementing!
You can revert back to using the old TinyMCE editor if you wish. To do so, click on your name at the top right of the screen, select 'Preferences', then User account> Editor preferences. Then you will have the options that were previously available.
You will find further information at https://docs.moodle.org/311/en/Atto_editor#Non_default_Atto_plugins