I don't think that a darker grout will take away from the hex, as looking at the first picture, the way the light is (or lack of) it looks like there is a darker grout on it now. I would be more concerned if you want a darker grout between the wall tiles, to highlight that pattern. It may seem busy with both a dark grout on white wall tile, and having the hex mosaic accent. I would go with a lighter gray or a shade of white. It will melt into the wall tile, and help to pop the hex. I would possibly use a little bit of a darker grout on the floor for cleaning purposes.

do a warm gray for the subways, and a darker gray for the hex. Or just used the gray, 4th in from the left, for all of them.

I did it with mine. warm gray for the subways, Charcoal for the floor

Polyblend Oyster Gray or Delorean on the shower and accent.
New Taupe on the main floor.
Here is my kitchen with Bright White on white subway. It fades in which I’m happy with. I just wasn’t sure about putting that Bright White grout in the hex accent in the shower. I can’t find any photo with white grout in that warmer darker hex.
I like your subway. It’s subtle. What color is it?