Just a note, this does take some elbow grease and time to actually do (one of my colleagues did it to her bathroom and it took her a few hours) but in comparison to actually regrouting your whole floor — this is a total time-saver.
Promising review: "I have 12-inch white tile in my whole house that is 20 years old. Even though I have kept it looking nice, the grout still was getting dark. I cleaned the grout and let it dry good 'til the next day then used the grout pen. It looks like it is brand-new. One thing that I found out was that if the tip of the pen starts to look worn you can just pull it out and turn it over — it is the same tip on both ends. It will take some time to do the whole house but it looks so great that I will just do a section at a time 'til I finish. One other thing that I did was run my finger down the grout line after I apply the grout pen then wipe down the tile line with a paper towel so no other cleanup was necessary." —Amazon Customer
Get it from Amazon for $8.99+ (available in two tip sizes).