The grout had become light yellow in color. Yucky.
I went to Home Depot and picked up a bottle of this stuff. Grout Renew in Bright White. I used a stiff, square tip 1/4-inch paint brush and went to work. The bottle suggests a tooth brush but I wanted as little mess as possible. I also had a roll of toilet paper near by. I used one square of toilet paper for each section I was cleaning up. A paper towel would be too hefty, the TP was just right. Once I applied it in the grout lines I then wiped along each side of the tile. It dries fast so work in small areas.
The "afters" are great! Looks like brand new tile installation!
It's a little back breaking to be hunched over for so long, but I just love the clean look.