Your tiler is talking nonsense. However,several weeks is a long time not to notice the problem.

Soften the grout with a grout haze remove (which is just a mild acid. Vinegar often works just as well.) Scrape it with a plastic (not metal) scraper. Scrub off any remainder with Scotch-Brite pad, soap and water.

You may have to do this more than once.

I can't image a glazed tile has changed color, What you are seeing is probably just grout residue.