We spilt ferrous based moss killer on our natural limestone patio tiles, leaving the most horrendous orange/yellow stain. The tiles hadn't been sealed so it was a well absorbed stain. We weren't sure if this would work but wanted to try before digging out the tile. I'd read the reviews about it going purple but it was still quite alarming, it also had a strong smell as it started to work. BUT it washed off completely and had completely removed the stain. Saved us a lot of time and money. The reviews were helpful for me so wanted to leave ours in case helpful for others.