This pen hides dirt, so a clean surface will help hide dirt, so make sure you clean your grout beforehand.
The grout is available in 11 different colours: white, winter grey, light grey, grey, dark grey, black, ivory, cream, beige, brown and terracotta. View all colours here.
Promising review: “We have 12” white tile throughout our house, and it's 20 years old. It looked fine, but the grout was dark. I cleaned the grout, let it dry well until the next day, then used a grout pen. It looks like new. I found that when the tip of the pen gets worn down, you just pull it out and flip it over – both ends have the same tip. It takes a while to cover the whole house, but the finish is so good I'll keep painting one section at a time until I'm done. One more thing, after applying the grout pen, I run my finger along the grout lines and wipe the tile lines with a paper towel, and no other cleaning was required.”—Amazon Customer
You can get them on Amazon for $8.99+ (they come in thin and thick tip versions).