I would use the gray from the counter.

If you are set on a white subway tile then I would do your best to match the veins in the counters to the grout color. If you aren't set on white, you could always break up the white lower cabinets/counters and the upper cabinets with a light gray color like pictured below.

We carry a taupe gray that I have linked below. Bright white grout and the gray tiles will add a much needed contrast between the all white theme. We have samples available in our Houzz store if you would like to check it out.

3"x6" Taupe Gray Glass Subway Tile · More Info

Hi---

Definitely light gray---anything darker will deter the eye from the beautiful marble counters. Hope this is helpful. Good Luck with your project!