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: “The white is a true bright white without any ivory or yellow. Do not count on this matching your existing grout. You will not be able to just touch up a few areas. Just plan on doing the entire room. As to usage, I was pleasantly surprised by how far these went. I did the entire floor of my large master bathroom plus a little of the shower with one pen. The shower floor has small hex tile with larger grout lines so that took a little over two pens. Some tips: shake very well when first taking out of the package and any time you start working again after a pause. Depress the tip several times onto a paper plate or directly onto the grout line to get the paint flowing. Press very lightly when going over the grout. The tips lasted almost all the way to the end of my project. If the coverage starts to thin, pause and depress the tip a few more times. I would do about a 1×1 foot area, then pass over it lightly with a damp paper towel to get any overage off the tile. It is pretty forgiving and you have quite a bit of working time before it dries completely on the tile. If the tip gets too ragged you can pull it out (I used the damp paper towel) and flip it around. This was not a quick task but it went faster than I thought it would. I was worried the white would look too bright, but it just makes everything look so much cleaner. Very happy with this product!” —Jenelle
Get it from Amazon for $8.99+ (available with a narrow or wide tip).