Thank you for your post.
Marble subway tile with grey grout.
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.
I went with tec standard white grout and it came out very yellow or antique white against the tile.
Especially when looking at it in the daylight.
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.
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.
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.
Still a very pretty look which we love.
White grout is a risk in bathrooms though as everyone mentions.
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.