Use the best color grout that works with the floor. Then do two grouted sample boards for the white wall tiles with the 2 different choices. Once you see how it looks then decide what grout to use on the walls.

For me, I would use white grout on the walls as I prefer grout to blend in and not stand out.

It would help to get better opinions if you showed what the floor tile and wall tile are and what the layout is ...

" For me, I would use white grout on the walls as I prefer grout to blend in and not stand out. "

Exactly, grout is functional, not a design element.

Thank you both - I agree on the white grout but they really liked the look of dark grout on white subway tile so that's happening. The question is, would it work to have a lighter grey grout on the subway tile and a darker grey grout on the floor, or should they be the same shade of grey... my feeling is they should be the same shade but curious your thoughts. Hex floor tile attached (and matte white subway 3x6 subway tile on walls)

No need to stick to one grout shade; look at each element and what is the best color choice for that.

Definitely use a darker gray grout on the floor.

The floor tile also has light gray, so that would tie in fine with using a lighter grout on the white subway.

I would do a board with the light gray grout and one with white grout for the subway. Show them how the floor becomes the main element when it isn't competing with rectangular light gray grid lines on the walls. I definitely would not do dark gray grout on the walls.

We have white hex on the floor and green subway on the wall—never even considered matching the grout. We used dark gray (Custom Pewter) on the floor and light gray-green (Mapei Mint) on the wall and are pleased with both. i did have to test a lot of colors with the green tile to find one that didn’t pull pink or yellow, though.