Grout getting darker (but it's not dirt)

Hello there I had a similar problem on my last job I was just painting the bathroom ceiling but the client asked me why the grout was dark and almost black in places after only 2 months of new tiling and grouting, I removed all the grout and left for a few days to dry out and then I re grouted and so far it seems to be ok, I think it could be moisture in the wall behind and the tiles and grout trapped it in causing a mould to form, it could be similar to your problem, maybe remove the grout and leave for a few days or a week to breath and then re grout the area

Av seen this problem before, more so at the base of the shower tray, so I removed the defective discolored grout, which was mid grey, left to dry for 24 hours, reinsert new grout, then put a sealer on it. Had no problem since.

If I was a betting man I would say moisture has penetrated the area, it would be best practise to remove Grout in that area and alow to dry out. However moisture maybe coming from behind so this may have to be investigated. Hard to give a definitive answer when the problem may not be as simple as portrayed.

My advice is 1st to look at what products are used to clean the floor it’s seems house hold cleaners are used which maybe why wet areas are cleaner as they receive more water which could be washing the products away Normal house products will discolour your grout and make it darker Use correct products for cleaning surface and always wash down with clean water