To me the Snow White is reading creamy. I don't think it's a good match for the Carrara worktops. A bright white would be the better bet or I do really like the platinum

However I also like the rolling fog and the Cape Gray

I always suggest doing a mock up board if you are unsure of grout colours. There have been so many threads on here where people are upset because the grout colour has been completely different from what they were expecting.

My personal preference is a bit of contrast with white subways, but as somersletlass says, make sure it is grey and not cream. I think we used platinum, but it's been a while and I can't really remember. If it is, it is a true pale grey. some of the greys look pale on the chip, but in real life are quite dark.

Your kitchen looks amazing and I can hardly wait for your reveal pictures. Keep us posted!

Yesterday was grouting day for my light green glass backsplash tile. I realize you have white tiles, but here's what I did. I bought boxes of both, and tried both colors with scrap pieces of the tile next to the countertop. For my particular installation, the consensus was that Snow White looked better because it matched the color of the countertop.

Personally I like a grey grout that mocks kitchens from the 30's. I am doing an entire room off my kitchen in white 3 by 6 subway with gray grout. Also doing alavendar tile backsplash in the kitchen (tile is 80 yrs old) with gray grout. If you want your wall to disappear then matching grout to tile color is good. If you rather have a vintage look then consider gray. Mocking up like daisychain suggests is a great idea. Good luck. Big decision.

Mock ups are an excellent idea. Look at them in different lights at different times of day.

FWIW the whitest white grout is Avalanche by Mapei.

Be sure to seal the countertops before grouting the walls as the grouting process can stain the marble