The GUI shows and hides various settings depending on languages installed and other regional settings but everything 'should' be controllable by macros regardless of what the GUI shows. I don't see the options that Paul showed in his screen grab but there are some vba commands that appear to have potential.
Code:
Options.ShowDiacritics = Not Options.ShowDiacritics
Options.DiacriticColorVal = wdColorBlack 'or RGB(0,0,0)
ActiveDocument.Range.Font.DiacriticColor = wdColorBlack
Note that the first line works to toggle the visibility of diacritics on my machine but the next two lines don't appear to have any visible effect on my machine but may work for you. You may have more luck than me (my diacritics were already black and fiddling with those values didn't change that)
EDIT: Ahh, I see Paul has found the right setting. Now I can dial in different colours for diacritics or not.