I use straight vinegar - the cheapest you can find. I do one area at a time. I pour the vinegar into the grout, leave it a short time, scrub with a brush then wash that area. Go onto the next area. It bought the grout back to it's original. When completely finished give another good wash and most of the vinegar smell will disappear quickly. It is a bit of work but worth it in the end and no toxic chemicals in your home.