I'd find a gray that matches one in your marble. It'll blend in more smoothly with the silvery color, whereas much darker or (Gawd forbid) white would just look like an 80s shower. Can't wait to see your kitchen!

Amen to what TX says - I, personally, like dark charcoal. I agree w/TX on the background on your lighthouse showing how well the black is. The liquid charcoal at Lowes for coloring concrete is so dark it's almost black, and that's what I'm using lately on everything, when I don't have black. This w/probably be the one I use on everything in the future. All that to say this: You get to choose. If you don't like it afterwards, there's the good ole acrylic paints in many colors. You can change it as many times as you like.

I agree with the rest. If it were a checkerboard, that would be a different story. Black grout would be the thing-- either way-- black or white would look exactly the same, believe it or not. Black would be easier to keep up. but being that you have white tiles against white tiles, the black would be TOO much of a contrast. Go with a medium grey, something like Custom Delorean grey, or Mapei Pewter.

Don’t know if it helps but here is a pic of square Mallorca white with Prism grout in Bleached Wood...