Hello partners and a Happy 4th of July to all.  I have a friend/ customer with a small black slate shower floor with 2" x 2" tiles.  Some of the tiles randomly have turned gray; and, in most cases, not the whole tile.   I was there about 6 weeks ago and stripped the floor to get it down to the state and be on the safe side  .  later I applied some color-enhancing sealer (penetrating), but within a week the gray tiles appeared again.  It is not efflorescence in my opinion, but the finish seems to have worn off.  I went there today to clean the floor and it will sit for 2 weeks so that it is totally dry.  I have never used a topical sealer, but I was wondering what you believe is a good approach to this problem.  Sorry no photos.