It’s difficult if not impossible to clean tile grout lines that have gone from white to brown or black. That’s because the dark areas are actually mold/mildew imbedded deep in the grout between tiles, not just on the surface. 
Cleaning mold/mildew off tile grout lines is the subject of a recent Expert Q&A in Atlanta Home Improvement. In it, Matt Hoots, CEO of SawHorse, Inc., offers this advice for making your tile look new again:
“The first (and easiest) is to prevent the growth in the first place. Mold/mildew tends to grow on surfaces that are moist and also have organic material that acts as a food source. Keeping it dry may be a challenge, but eliminating the food source is something that can be resolved by more frequent cleanings. If you are in the process of designing a space, consider products that are antimicrobial such as self-cleaning tile. To treat mold/mildew stains, one popular solution is to use a baking soda-and-vinegar paste. This is a green solution since it does not have any harmful off-gassing like typical detergents that can make you sick.”
Do you have any other solutions? Let us know by leaving a comment below.