Shame that Multiview can not search from linked documents, could be easier to find. But the place you should be looking at is MUI/MUI_Text - MUIA_Text_Contents in Includes & Autodocs.

Quote:

MUIA_Text_Contents, "\33P[778833]Some text here" sets direct RGB color value. You can embed color coding anywhere in your text and of course it is not limited to TextObjects but it can be used in most classes (listviews for example).

To display custom background you can use MUIA_Image_Spec formatting (IIRC) used in MUIA_Background tag, however most likely it might work better if you use custom class and render it yourself. You can also install window backfill hook but not sure how well it works in MUI. It has been about ten years when I had to mess with it even remotely in MUI applications.