I wouldn't go for contrast. I'd go with a urethane(Bostic) or epoxy(Fusion) grout.

I wouldn’t go for contrast with the wall tiles either.

Your tile and color choices are awesome!!!

Tell your installer to use either the Flexcolor CQ or the Kerapoxy line from Mapei to ensure the grout is stain resistant.

The only worst way than to selecting colors off a computer screen is to select them verbally.

Fsal. try Mapei Warm Gray for your marble and the black hex (if you want contrast) use the FlexColorCQ. it's a urethane, stain proof grout. (the installer has to know how to use this grout. it's not like regular grout.) I have it in my bathroom w/my marble. You might want to use Natural Gray for the black tiles.

The black herringbone shower tile is gorgeous. I'd use a medium gray/charcoal color for that one. I used the Charcoal w/my black marble hex tiles. I used the Black grout for those cement tiles on the wall, and the white subway tiles were done w/the Warm Gray.

I also used the Black on my marble basketweave tile. Warm Gray was done on the marble floor. here is a close up pic of both if it helps

The frost is OK for the penny rounds.

Again, use the Urethane or Epoxy based type of grouts, or if using regular grout, use a grout sealer periodically and do not clean w/bleach based product.

The laticrete Silver Shadow is also a nice choice.

I dislike the last pic w/ that Mist color. if it looks like that in person, it's too warm for those gray tiles.

have your tile guy do a sample board of the grouts you are liking. he can grout a few of each of the tiles w/your choices, wait for it to dry and then decide.

sorry. don't like those for your niche. not w/that beautiful floor. what if you did a solid black marble or black/white marble for the niche? I did this one for mine

those glass bars really don't jive w/the classy marble.

maybe a white thassos marble ? this would look nice in the niche.

I'd skip the high contrast for the hex. Your tiles are lighter than your example, so the lines will show up just fine even with a closely matching grout. I'd pick a shade in the tile.

How many places do you want to demand attention (in the bath?). I'm perplexed by those who choose multiple tiles/patterns for that room. Gets overwhelming, i think. And, in all cases - do you want the tile to be the 'star?' Or the grout? And very light grout is typically going to mean more housekeeping attention....

what is going on the shower walls if that marble is on the floor? is this the herringbone black tile on the walls? I don't know how many bathrooms you're doing.

if doing a white subway and that marble is on the floor, then do the same marble in the niche. I wouldn't do the black subway tile in the niche unless your shower walls are the same black subway.

ok. i see. then use a different material instead of those glass bars. to me, the glass is more modern, whereas everything else you have is more traditional. what if you used a smaller black porcelain mosaic?

it is! they have a 1" and a 2"

https://anttile.com/product/1-pure-color-pattern-triangular-glossy-black-porcelain-mosaic-tile/

home depot has these. Merola

Fsal, get yourself the Schluter Jolly trim to go around the niche. (i used it in mine in black if you want to see it). it's a much cleaner look if you can't use bullnose tiles.

thank you. (i used 6 different tiles in here. some say overkill, I say, 'nah'.)

I used the black Jolly around the subways and the niche

and just because that wasn't enough, I used the gold jolly on the inner part of the niche on the back portion ;)

Beth-I always love reading your tile advice. OP-I dont care how they advertise the stain resistance of a grout. They all freaking discolor, especially in the shower!!

Fsal,,,i've never seen that one. 15 years? wow. I would call your company where you bought your tiles and see if they ok that sealer. I'm not familiar w/it so I don't want to advise you.

Sealers Gold, 511, aqua mix,,,are all good. I don't believe the one I used was water based. on my marble i did,,,Sealer Gold. on the cement I used something different. it was an old bottle I had for awhile. still worked well. can't buy it anymore I think.

these will work

if you like the look of the tiles when they're wet, use an enhancer sealer. it will enrich the colors. don't use a shiny finish or wet look.

Fsal, when you posted this last pic of your awesome tile I told myself I wasn't going to harp on this again, but yesterday I finally gave in and used soft scrub with bleach in my stupid (honed) marble shower. Even that isn't enough to get the grout back to a semblance of its original (light) color. would kill to turn back the clock and get dark grout.

No traffic bathroom floor area

Post scrubbing shower floor same grout:

eye level:

i bet the tile guy loved you! nice work.

pick a medium gray grout. not as dark as the dark tile, but not as light as the other.

grab some leftover tile and do a sample board.

maybe something like Earth or Chateau, Oyster?

he did do a pretty good job. great minds think alike. I had my flooring guys continue the pattern up along the base portion of our platform bed. (if we ever move it has to stay!)

fsal, it's natural acacia. I had acacia in my hallway and entry, so when it came time to redo the bedroom I got the same type of wood. I wanted a herringbone but finding pre-milled boards in something other than maple was difficult. Oak sealed on site was an option, but an expensive one.

I searched for almost 6 months before I found this. the boards were all cut and T&G on all sides. (left boards and right boards. yeah, it was not a fun install!)

I love my floor!