Be sure to clean your grout beforehand since this pen isn't a replacement for a cleaning! This will help you fix any stains that won't come off.
There are 11 different grout colors available: white, winter grey, light grey, grey, dark grey, black, ivory, cream, beige, brown, and terracotta. You can view all the colors here.
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.