There are no helper methods for it yet, but I think it can be done with the raw API (though I haven't tried this myself yet). FPDFPageObj_CreateNewRect() and FPDFPageObj_SetFillColor() should do the trick. Take a look at fpdf_edit.h.

Also note that the current insert_text() method has problems and will be removed or replaced with something simpler on the next major release. It currently attempts to insert text using an external text shaping engine (for foreign writing systems) but that is quite complex and the implementation has issues. As long as you don't need to support Hindi etc., you're better off with FPDFPageObj_NewTextObj() (or FPDFPageObj_CreateTextObj()) and FPDFText_SetText() until I have re-written the text insertion API.