Hi, I have recently tiled a bathroom in a 9mm glazed Pietra serena lev 60x30 polished porcelain tile. I used a black bostik technik TGS grout but unfortunately it has left behind staining that I cannot remove, I have used HG cement grout film remover and vinegar but to no avail, I can’t seem to shift it, what exactly would remove this dye staining from the tiles, the last thing I would want to do is to completely cover the tiles in grout to match existing staining or have to remove the tiles completely and start again, any ideas would be really appreciated, thanks so much