Hiya
Are you sure they're porcelain? After being hard pressed, porcelain tiles are fired at 10000+ celsius, they're fully vitrified with a very low level of water absorbtion, they're almost impervious to water. They're harder, stronger and more durable than natural stone.
Porcelain can take on the colour of cleaning residues. Unglazed surfaces sometimes assume the color of the cleaning solution.
This might help as a basic cleaning spec.
Saturate the floor with cleaning solutions and warm water, agitate and allow to dwell for 5 to 10 minutes. Do not allow to dry. Do it on small areas at a time.
For more difficult stains, scrub the cleaning solution with a rotary machine equipped with an abrasive pad or stiff nylon-bristle brush. For small domestic, use a deck scrub brush.
Extract soiled cleaning solution and rinse the surface of the porcelain tile with clean, clear water with a mop or a wet vacuum.
You might have to use a more aggressive cleaning method if the unpolished porcelain tile is heavily stained.
I used Prochem product (QMT Stone Clean) on a quarry tile floor which had a lot of white inflorescence coming up through the tiles. Both customer and I were pleased with the result. On another job with similar tiles had kitchen fat/oil on top of tile sealer, used Ultra Pac Renovate, also very happy with result.
Grant