If you want the grout to fade into the background then white - for peace of mind grey. The grey grout kind of frames the tile so it becomes more of a co-start than a supporting character. I would never ever consider white because I would loose my mind with the maintenance issues. Make this another vote for grey!

Caligirl, I don't have tile in the shower but if I did I would have probably done subway tile with the same delorean gray grout. I have always loved subway tile even before it became really popular again. It was used during the same time as the hexagon tile and was often used together so you can't go wrong with it. If we had the time in our tiny master bath I would have either shiplapped the walls to just under the switch plates and added a ledge or done the white subway tile to the floor with the gray grout. I'm almost done with the master bath and I have to say it is coming out so beautifully can't wait to show it. Good luck on whatever you go with.

not contributing anything new-darker, or rather contrasting, grout when you want to emphasize the shape or the pattern of the tile, blending-close to the color of the given tile grout-if you don't here's another thought for you..light gray/bone will be probably fine. the darker you take it-the stronger the contrast. which might be amazing looking and all but take into consideration-not everybody is doing great with high contrast, especially very dark and very light together, colors and lines interchanging frequently as in with the grout and tiles..it makes a lot of people dizzy-not good for a space like a bathroom where you'd rather not feel dizzy if it was for your own enjoyment and you handle high contrast well-go for it if it's for resale-take into consideration a pretty big amount of people that will find it hard on their eyes i also wanted to go with contrasting grout..fortunately i realized i'd be the first to fall in my own shower, surrounded by this striking geometry again, if it's just a tad darker color though-it will probably look cool and still be fine with the majority of the buyers

Thanks! And how exciting! I’m in the same boat and never thought grout would be what stresses me out so much. Do you mind if I ask what color tile it’s going with? We’re doing a mosaic type matte black backsplash in our kitchen and thinking to pair with either sterling or Silverado. Also scared the contrast will be too crazy. Congrats on your new home!!