You can change editor from your user profile. Default editor is Atto - the old default editor TinyMCE has color options.
Or you can add 'fontcolor' and 'backcolor' to Atto's toolbar https://docs.moodle.org/28/en/Text_editor#Toolbar_settings (only a few colors)
or install https://moodle.org/plugins/view.php?plugin=atto_morefontcolors