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 just bought my first house and because of COVID I have not had the funds I wanted to fix it up. This pen was cheap and the backsplash in my new kitchen was an eyesore. I wasn't expecting much because of the price but I am absolutely stunned. My kitchen looks so much better!It was easy to use, no odor, and quick! I did two coats because the color was dark brown. Some places I did more but I didn't mind doing it because it was actually really satisfying. I used one pen for the area plus some. Definitely recommend this product!" —Kim
Get it from Amazon for $8.99+ (available in two sizes and four colors).