Cleaning tile grout can be difficult and tricky depending on the nature of the stain. Grout is porous therefore some stains soak in and are difficult to budge. Most tile manufacturers recommend that you use a cleaner with a neutral pH. I know this sounds funny but there are cleaners that fit this description. My experience tells me that they are usually ineffective. Dominion Restoration makes a tile cleaner called Pro-tile 700. It worked for me around my house. The term clean is subjective. What looks clean to me may not seem so clean to you. A standard for measuring clean on tile is coefficent of friction. You may try this web site for more information www.nfsi.org. In closing I will recommend that once you get the grout clean you should seal it.