Use regular bleach (like Chlorox or a store brand) for cleaning grout. Dilute it 50/50 with water and put it in a spray bottle. Make sure the room is well ventilated before you start spraying the mixture on the tile and grout. Be careful not to spray it on your skin, your face, your clothing, shower curtain or any fabrics as it will bleach out any fabrics it touches. Do not inhale great amounts of the mist.. Household bleach is oxygenated bleach. Products like Oxy-Clean are unoxygenated, which is why they are safe for colored fabrics. Regular household bleach will kill mildew that's grown on grout.