Try putting rubbing alcohol. Put it in a well-marked spray bottle & spray it into the grout. Let it soak a few seconds then spray again, then absorb with a paper towel. You can also try the stronger 90% rubbing alcohol & also try fingernail polish remover. Use the stronger one that's NOT made for "artificial nails". Also, they sell professional ink removers (for janitors, etc) on the internet. (they work wonders!) Once it's clean, be sure to seal your grout!