Grout pens do not work, I have tried them and they are totally useless in any case. What you can do is re-grout the tiles if there is a enough space for the grout to take place and set. I have done this with some tiles that I wanted to add color to. I would recommend that if you decide to take this step that you make it a darker color so that you do not have the same problem later.

This should not take more than a few hours to complete provided you have knowledge and prepared the tiles for the grouting.