Paperboy: My sister found a product on one of those TV purchasing programs but "it really works." They have several colors to choose from and you do have to order via their web site. I have not used the product but she used this stuff on her grout lines on the bathroom floors, showers, etc a few years ago (four +) and they still look new.
Grout Finish So long as your spots really aren't mold, this will probably do what you want.