#115 Platinum is a very nice color that will compliment the veining in the tile and mosaics. If you want another color that may blend better with the tile but still have the gray hues try #544 Rolling Fog.

i'm doing warm grey by Mapei with mine (pic is of that grout, not my tile)

delorean gray is another one that goes well w/the marble.

White. And please do not mix porcelain "faux" marble with the real stone.. The difference will be very obvious, once installed. If cost is the issue, put the porcelain large format on the bath floor, and shower walls........and find a deep gray mosaic solid porcelain 2 " mosaic, and grout to match IT and its placement on shower floor only.

Or splurge for marble, on the bath floor : )

In either case, you want to see the veining, NOT the grout.

agree about mixing the two different materials. disagree on the white grout. carrara is not a bright white. putting white grout w/carrara marble looks off. carrara white is a grayish white.

here is a bathroom I tiled in cararra and used delorean grey 9on the walls and on the floor) . delorean has more of a lavender undertone. I'm using the warm gray on my next one. and the eye really doesn't see the light gray.

here it is with white grout. to me, this is all you see.

but the white with this hex against the white cabs, looks better. it depends on your grout width, how white your marble is and what else is around it.

The tile in the shower is all Italian carrera so because of the cost I don't want to mess it up with the grout. I believe the first pic you sent is what I want