Hi Kim,
Xperience uses CK Editor which can be customised via the administration application file system. Just a warning though as this will mean you're updating CMS files - something generally only done if absolutely necessary. Changing CMS files has the potential to cause problems if you're not careful and can also affect future upgrades. So proceed with caution.
Having said that, the CK Editor configuration files can be found in:
..\CMSAdminControls\CKeditor\
This article should help you achieve what you're after:
https://ckeditor.com/docs/ckeditor4/latest/features/colorbutton.html