Hi Ray, good tip.  I had grout that turned a darker color in some places. No one could explain it.  The only thing that I could do was recolor it.   I used a colorant from C-Cure  (who was the maker of the grout) and had to redo all of the grout lines.  I simply followed the direction and used an old toothbrush and it was good as new.  It is also a sealer too. 

This is a good option if you just do not like the color of your grout instead of getting rid of the grout.  If you want to go lighter, you have to do 2 coats. anna hackman www.green-talk.com