This would be totally a personal preference thing. Some people prefer the contrast on white subways as the "classic" look. Others want the grout to blend in to the field so that you really just see a wall of white.

Do you want to see the pattern of the subways or not? Plug into Houzz some search criteria and see what comes up. I know that worked for me when I was looking at white subways with silver shadow grout.

For example:

[Traditional Kitchen design[(https://www.houzz.com/photos/traditional-kitchen-ideas-phbr1-bp~t_709~s_2107) by Salt Lake City General Contractor Tiek Built Homes

Versus:

[Traditional Kitchen design[(https://www.houzz.com/photos/traditional-kitchen-ideas-phbr1-bp~t_709~s_2107) by San Francisco General Contractor Dijeau Poage Construction

I plan to use white grout with the subway on the walls and gray grout with the hex on the floors in my new build. I have seen it this way many times and it looks great. That said, I think the gray grout looks really nice with white subway as well. Just a matter of how much emphasis you want to put on the shape and size of the tile - not a maintenance issue on the walls.

I usually specify white on the walls and grey on the floors.

I have hex flooring, rittenhouse subway on walls mapei silver on both

We just had white subway (rittenhouse) installed with white grout on the walls. And then unglazed hex on the floor with silverado grout. It looks great. I personally think white subway with white grout is more classic looking, but I like the look of a white grey as well.

I think it depends on your overall aesthetic. If you are trying to have your bathroom look original like we were, I would go with the classic white. But if you are doing more of a mixed style in your bathroom with modern touches, I might go with a darker grout. You can still see the brick pattern with white, but it is much more subtle.

Of course in our case, we were also dealing with a black subway chair rail and black bulnose. I rightly thought a non-white grout would look funny.