Well, whaddayaknow, I found a pic of the back hall bath. It's pretty hard to see the floor grout, but maybe you can tell that the hex pattern is somewhat indistinct compared to the other photo. Part of the issue, I think, is this tile is more of an off-white than the two-inch, so the light gray grout actually looks white next to the tile.

Should the grout be the same color?

Not necessarily.

Will it look weird to use a darker gray on the floor and a lighter gray in the shower?

Not at all.

Any recommended colors for the shower and floor tiles along these lines? I've been getting stuff at Lowe's and they carry MAPEI

Warm gray for the subway tile, and silver for the hex, if you lke a darker grey for the hex. Silver is actually a medium grey, but it'll appear alot darker with the white tile.

Our bathroom isn't perfectly square. I think a colored grout seam that is uneven will be really obvious(that's my understanding where you would do any flubbing). Can you grout just the seam white? Can you skip grout and caulk it? Any suggestions here?

Grout everything on the floor the same color. it'll actually show up more if you try doing what you suggested. It'll literally draw attention to it. What you want to dpo is make sure that you're square to the most obvious walls from the door. Especially with the hex, it won't be quite as obvious as you think, unless it's WAY out, and then nothing you can do will hide it, even using a blending color grout. One other thing-- that joint where the floor hits the walls should be caulked, but still caulk it with a caulking that matches the grout, both color AND texture.

Does anyone know what the measure is for hex tile and subway?

For subway, use unsanded, and for the hex, use sanded.

Changing grout colors from one surface to another isn't a big deal. There have been occasions where I've changed grout color within the wall itself, usually when detailing an inset. A mosaic panel, a deco band, etc.

Wow, you guys are great-I am very appreciative! Two follow up questions. For Bill V.-Thanks for addressing each of my questions! Re: my colored seam issue- I am also concerned about having an uneven colored seam where the subway meets the tub.

See link inserted for what first alerted me to this issue.
I don't know what they used for white seam at the tub. Can you grout just white at the bottom? Any thoughts?

For nutherokie, Thanks so much for pics! Your back hall bath is closest to my project (beautiful master,though!). I love the subtle level of contrast for your subway tile in the hall bath- perfect! Your consideration of a darker color for more contrast on the hex floor is exactly what I am wrangling with in the planning stage so I appreciate your sharing. You've got a great looking shower and floor so you can't go wrong either way whatever you decide. Would you be willing to share brand and color of grout in master bath floor and hall bath wall? I'm adding my email to my profile just in case. Thanks all!

Here is a link that might be useful: colored seam issue

I want to bump this up. I am having the same conundrum when remodeling my period reproduction bath.

Wall tile: Subway ceramics in Avalon (antique white)

Grout in the showroom was Polyblend Urban Putty (an aged very light tan). However I don't know if they did any custom mixing.

All the same as this pic, but no accent colors.

http://st.houzz.com/simgs/fb72f14f04d2f9fa_8-2700/home-design.jpg

Floor tiles are 1928 original white mosaic squares set with what is now a very light gray grout, but a test of the sealer shows that the sealer will deepen it considerably. Here is a pic of it before the sealer (I will take a pic of the sealer in the AM)