I was removing some silicone with a multi tool and got black metal marks all over my white glossy tiles. I just couldn't get them off. I tried Ajax, CLR, Magic Erasers and Jiff and it just wouldn't budge. Someone suggested Bar Keepers friend, but I didn't have any of that in the cupboard.
BUT I FOUND A FIX and it is AMAZING - Citric Acid
Citric Acid. You can buy it from the supermarket for about $2 in the baking aisle. It comes in a little tin. I had some in the cupboard so I gave it a go. I just sprinkled some on the mark and wet it with a few drops of water. Left it for about 3 minutes and then wiped it away with a wet cloth. NO SCRUBBING AT ALL, just a wipe and it was gone. I did all the other marks. For the wall where I couldn't get it to stick I just folded and wet a little square of paper towel, sprinkled it liberally with citric acid and stuck it to the wall. Again, 3 minutes later and the mark was completely gone.
It didn't take any of the shine or glaze off the tile either.
Not only that, but the grout that it dripped onto was sparkling white again.