Word and LibreOffice will highlight Fields/Reference Marks on your screen to indicate that the text is automatically generated. This can help you avoid accidentally manually typing in the fields (to edit the text shown in a Zotero citation, see Customizing Cites). These highlights are only shown on screen and won't appear if you print or save the document as a PDF.
You can change the settings for highlighting Fields/Reference Marks in your word processor:
See also Citations Highlighted.