Use a non sanded grout to protect both the marble and the grout from discoloring too much.
Marble subway tile with grey grout.
As a backsplash installer that s all i do i must agree with you that pale grey is indeed the most popular grout colour for standard 3 6 white subway tile.
White grout is a risk in bathrooms though as everyone mentions.
Especially when looking at it in the daylight.
Five years ago mosaic on mesh was the most popular option for a backsplash accounting four about 75 of installations.
Now when you go with a black tile and a white grout you are going to see that visually like with white tile and black grout things start looking a bit busier and you do make out the shapes of the tiles more so than you would if it was all tonal.
Still a very pretty look which we love.
Soft gray walls are accented with white subway tiles finished with gray grout lined with a black washstand restoration hardware maison powder room vanity topped with honed carrera marble fitted with an oval sink and a polished nickel gooseneck faucet as well as an inset framed medicine cabinet illuminated by two light sconces.
For anyone checking back on this i am having a new house built and had a carrara marble 3x6 subway tile backsplash put in the kitchen with absolute black granite.
Thank you for your post.