Chilli if you have got any T-cut in the back of the car, black stain a spare tile with the said grout and give it a try by rubbing the tile with a tea cloth.
You never know, I can't help beyond that. Good luck!
First of all the tiles were covered in a plastic stuff and wax, so we took the plastic stuff off before we layed them, we then got a tile cleaner from Topps tiles called FILA, then we sealed them with the FILA sealant.
Chilli if it is surface stain it should come off. However, if it has penetrated you will need a deeper specialist cleaner. I would hold fire for a while longer untill you get further replies. 'Dave' I'm sure will hit the nail on the head.
What ever method you use i would do this on a spare tile.
As for the T Cut i would be careful that the T Cut doesnt penetrate the tile and make matters worse. Never tried it so i dont know but best test on a spare tile 1st.
Sorry to hear your dilemma, did you seal them before fixing edges as well as face? some of this stuff can take in a stain from the joint, if edges are not sealed, causing a picture frame effect. there are some products that could help. Maybe some pics for more detail