Thank you for your post.
Marble subway tile with grey grout.
White grout is a risk in bathrooms though as everyone mentions.
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.
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.
Especially when looking at it in the daylight.
I went with tec standard white grout and it came out very yellow or antique white against the tile.
The light gray is also a fine choice but keep in mind that it will darken and maybe not match the tile color quite as well over time.
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.
Light gray kitchen accented with gray wash wood floors boasts light gray shaker cabinets donning oil rubbed bronze hardware and a white marble countertop holding a farmhouse sink with a copper gooseneck faucet beneath stacked wooden shelves flanked by gray upper cabinets mounted on a white subway tile backsplash contrasted with dark grout.
Use a non sanded grout to protect both the marble and the grout from discoloring too much.