Had a client call had a new floor installed (wood ceramic style) using a dark walnut grout by Tec. The installer has cleaned the floor multiple times  and its been down for over two weeks now. If you keep wiping the grout the color will transfer onto the rag. This is what the home owner has told me. I haven't seen it yet just wondering if anyone has come across this before.
Thanks
Justin Cameron
JMC Stone and Tile Care
Justin@JMCStoneandTileCare.com
951-972-6997