For large areas of grout discoloration, spray with hydrogen peroxide, let sit and rinse/wipe. Note that hydrogen peroxide can bleach color, so do the test first. For stubborn stains, apply a 2:1 paste ...

It fills the gaps between the tile, helping to prevent moisture from getting into the subfloor or the walls behind the tile. The compound also helps keep the tiles in place, ensuring they don't slide ...