Hello partners

I received an email today :

My tile guy put in shower floor today, natural stone pieces, it was a very light grey, he grouted with a silver unsanded grey grout. The tiles now look a brownish tone. We are wondering what caused this, and we can do to fix it.

I've seen iron stains that have developed over time from too much moisture. This sounds like it happened instantaneous.
Any thoughts?

Thanks Chris

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