A couple possible causes could be either latex leaching or efflorescence. We experienced efflorescence - it's where the minerals in the subfloor below the tile rise through the grout and leave a white residue on the surface. Our grout turned orange in several spots from the iron, and still has sort of a white-ish residue.

Thanks Pip! You are referring to the hex marble floors, correct? I love both your bathrooms and I know it probably drove you crazy when the grout stared to turn. Did you all try any remedies or just live with your grout discoloration?

Yes, the efflorescence affected my marble hex floors in our shower. I read online that a mixture of vinegar and water can alleviate the problem, but I didn't try that on our marble. I took a toothbrush to it with some very hot water and scrubbed, and it's definitely better, but not completely gone. The orange stains worked themselves out after a few weeks, but the grout still shows some white spots you look closely at it.

Just wanted to report back~ the installer believes this is thinset that has risen to the top of the grout. He is going to dig/cut out the thinset spots and regrout.