I have no idea, I'm sure there's a logical reason. Anyone?

I would do gray black grout is hard to keep looking black.

Why are you doing squares? The hex won't show if the wall is out of square, the square tile shows it immediately. Are you using cove base in the shower like below?

I just re-read my original post and our project is poorly described, my apologies! So we are doing 4 x 16 glossy white subway tiles on shower walls and the basic white mini (1 x 1) hex on the floor, including the shower floor. Exactly as the first picture above, thank you. There are no square tiles being used anywhere.

Building a master bath and completely remodeling our main floor bath has my head spinning with all the variations and choices!

It sounds like the reason people use gray over black is that it's historical, and also easier and more forgiving to install and maintain than black. Thanks everyone for that information, we will definitely go with a shade of gray!

The look is 100% unforgiving of poor framing under the tile and poor tile laying practices. "Simple looks" are a lot harder and much more expensive to achieve than ones where there are multiple details to fool the eye. Be sure to vet your contractor thoroughly. Rebuilding a bathroom needs to involve a couple of days of just adjusting the framing to be perfectly square and plumb. It is the foundation to the entire look. The "historical" look can quickly get off track unless high skills and high priced labor are involved.

Thanks for the advice, live_wire_oak.

I do trust our contractor and have worked with him in the past.

Black grout is not readily available at hardware stores. charcoal and darker greys are. So most people don't want to wait for special orders to come in. That is why I believe people choose charcoal instead of black.