The most common reason for grout turning white is known as efflorescence. This is a reaction of moisture with the grout and it can happen in a variety of ways. First, moisture can rise from below the surface and stain the grout with salt and mineral deposits. Also, most people tend to use black grout in a shower or on a bathroom floor near the bathtub. This will expose the grout to moisture and splashes of water which can discolor the grout over time. One other reason for grout discoloration is harsh chemicals used to clean the grout which leave a residue.