I always use grey grout for the Roman mosaics, never white or Ivory. These lighter colours tend to ‘separate’ the tesserae (tiles) whereas the grey pulls them together allowing the focus to be on the pattern.
Another thing I do is to use the adhesive as a grout. Grout is, essentially a fine powder grout. When you wipe off the excess it should remian fairly level with the top of the tiles. Use the courser adhesive mix and when you clean it off it will remove more of the cement leaving a more ‘worn’ look.