We always recommend darker grey with the Carrara, accentuating the darker veins - maintenance is much easier for the clients too.
Whatever you do, don't use light grout on the floor! It was ALWAYS look dirty! I have a friend who made this same mistake. Looks gorgeous once it's first installed. But the grout will eventually turn brownish/yellow. Go darker!
i recommend a contrasting color for your mosaic tile to highlight the shape. if you went with white you wouldn't see much of the shape and it would blend into the wall. A light gray would highlight it really nicely I think, and you mentioned you will have other grays in your room too.
I'm having the same problem Dorian gray or platinum?
Definitely stay with a lighter color for marble. I hear it quite often, dark grout can sometimes seep into the sides of a light marble and stain it, causing it to look dingy overall. 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. White grout is a risk in bathrooms though, as everyone mentions. Use a non-sanded grout to protect both the marble and the grout from discoloring too much. The light gray is also a fine choice, but keep in mind that it will darken and maybe not match the tile color quite as well over time. Make sure to seal the tile once a year, especially on a bathroom floor, and seal the grout once every 10 years - people tend to forget that one!