Here is a picture of the tile

If you are looking for a gray paint for the wall (regardless of how dark), your choices will be leaning toward blue gray, violet gray or green gray. Green gray will not work well with the carrara marble so that will leave you with the blue and violet undertones. The carrara marble essentially is a blue gray color so that would be the obvious direction. Also, you didn't say what color the adjoining room is (presumably the master bedroom). Maybe your paint color could relate to that room somehow. If you would consider white, I'd just repeat the vanity color or look at SW High Reflective White.

We used SW crushed ice in our bathroom. it all depends on your lighting, natural and artificial. Make sure your lighting is installed and you have the bulbs put in that you are going to be using. Then paint your samples.

In our bedroom we used SW Silverpoint. It’s a light gray with a little bit of a pale green cast to it.

I was just playing around with the fan deck again. I think Big Chill might work. The master bedroom is a white from Ellen Kennon.

The bulbs and lighting are all in now (not when the other picture was taken the old yellow lights were still in). Big Chill is on the same strip as Snowbound.