For anyone checking back on this i am having a new house built and had a carrara marble 3x6 subway tile backsplash put in the kitchen with absolute black granite.
White marble tile with white grout.
The contractors did an amazing job of tile placement and we ended up ordering more marble tiles than planned because some of the ones we got were chipped or had a spot that looked like dirt etc.
If you re tile is white then go with white grout but if your tile isn t white and you can t match it exactly then white will just look awful.
For the white subway tile on the shower walls i selected a white slightly off white grout and for the marble basketweave floors i selected the closest color match to the color of the stone.
Jules we just had white marble flooring installed with black granite tile in a checkerboard pattern.
With white you ll also see more staining from water.
Use a non sanded grout to protect both the marble and the grout from discoloring too much.
White grout is a risk in bathrooms though as everyone mentions.
It will contrast even more than a darker shade because white reflects a lot of light.
Especially when looking at it in the daylight.
My vote is white it will keep the marble looking pristine and will darken a bit over time matching the look of natural carrara quite well.
Test marble thoroughly with dark grout by.
I went with tec standard white grout and it came out very yellow or antique white against the tile.