Nice choices! I love the pop of color you have in the accent tile. If you choose white or light grout color then the marble pieces will look more uniform since the tile will be close in color to the grout. The opposite holds true with the larger gray tiles - these tiles will "pop" because the lighter grout will have a contrasting effect. I think you're fine going either way. It's more of - which tiles do you want to "pop" more...

What tiles are going on the floor (if any)? If you have to choose a grout color for the floor then I'm always a fan of darker grout to help hide dirt/stains down the road because the grout will get darker with time if it's on a floor that gets used.

Thanks, FeatherBee. The floor is actually going to be a medium dark bamboo.