Create rich-text-editor add any styles you want (Remember to add class otherwise the changes effect the entire site). Add any styles/color you want - and than only than bind CMS RTE feild (Sometimes i put extra RTE next to the “bind” RTE and make this changes).
The docs about this issue:
Rich text elements can be connected to rich text fields in a Collection. When a RTE is connected, it’s no longer possible to select or edit text elements, except for in the CMS. That’s because the content is pulled from the CMS itself.
To style a connected RTE, it’s recommended to disconnect (unbind) it from the field, style the nested elements, and then re-connect (bind) it. If you just want to apply a class on a connected RTE, you can do that without disconnecting it. Learn more about dynamic rich texts.
Then follow the same article - post2 (I wrote this) - This idea works fine also on CMS RTE
If this not work for you i will update the original post (I am almost sure its fine).