Thanks, JENNR8R. That looks good and has the extra benefits of being inexpensive and non-toxic. I might be up to doing a few square feet a day, and by the time I finish the whole downstairs, the area I started with will probably be ready for the next round. I think I'll probably go with the Grout Refresh because getting a darker color would be a more permanent solution. However, it would be a good idea to get the grout as clean as possible before starting that process, so the baking soda and peroxide formula will be good for that.