Cleaning floor grout and tile grout can be done with SELIG COMMERCIAL GROUT CLEANER & WHITENER, an acid-based cleaner. One aspect about acidic cleaners is that they usually will not remove paint or varnish. Be careful not to get the grout cleaner on baseboards or cabinetry. If you get the acid on anything other than the tile, rinse it off immediately with lots of water.
 
Take heed to the instructions and warnings on the label because this product is strong and it works. Caution should be used when using this product so wear proper clothing; safety goggles and chemical resistant gloves will compliment your attire. If you have colored grout, test a small area behind the toilet and let it dry before using on the floor.

Apply the Grout Cleaner to a 2 x 2 foot area of the floor and let it set for three-minutes. Reapply over the stains and they should be dissolving as you work. Then rinse fully before working to the next area to remove any residue.

Once you are done, rinse the floor thoroughly to be sure to remove any residue that could leave a dull sheen on the tile.

When the floor is dry you can use a silicone grout sealer to keep the grout white and easy to clean the next time around.
 
Note: if you get this cleaner too close to chrome or stainless steel it will stain it. Not to worry because so far AUTOSOL metal polish has cleaned it and left everything like new.