If you want the grout to fade into the background then white - for peace of mind grey. The grey grout kind of frames the tile so it becomes more of a co-start than a supporting character. I would never ever consider white because I would loose my mind with the maintenance issues. Make this another vote for grey!

Thank you everyone! In that room I am going to do a light gray grout through out. I think it will hightlight the tiles. The vanity is white and I think gray grout will give the room some depth. Tinker, if you are up to it, you should show off your bathrooms, they are gorgeous! I do have a question. In the master I am doing a white marble floor w gray grout. But am thinking white grout for the shower subways as well as shower floor. It is a fancier space and I want the shower to take a backseat to the marble and other fixtures. Does that make sense?

I chose Warm Grey and am happy with it. I definitely wouldn't like it darker. Still wet - grouted on the left

The latest research suggests that sealing marble in a shower creates more issues than it prevents. Your tile professional should be aware of that, and the ongoing experiments. As well as the issues with using epoxy with natural stone. It’s very concerning hat you are posting here instead of working with him for answers to your questions. It suggests that he doesn’t know the answers, and thus is not quified to be doing your job. Further statements just confirm this. You might want to reread the specs on the Red Guard. More is NOT better. You are out of spec. And read the TCNA manual. Not following manufacturer’s installation instructions puts you outside of industry standards. Re vet any tile professional who would do that. Before you let him touch any of the rest. Because starting over now is better than staring o er after an installation failure. https://www.custombuildingproducts.com/TDS/TDS-104.pdf https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer