You could do grey or white, but stick with a solid color. (And make sure it goes with the white on the vanity.) No marble unless it's something VERY plain like Thassos. Your floor is too busy to mix it with the countertop you posted.

Maybe see if Kohler has an integrated sink/top that goes with your tile and vanity--they used to have a glassy one that might be nice with your tile. Keep in mind that some of the integrated and/or prefab tops will hang over both sides of a vanity and that might weird up your vanity positioning some and require filler against the wall.

The flooring you've selected looks great with the floor in the hall, by the way. What is it?

I have the same layout in my bath and all my samples looked much darker when I brought them inside. It was very frustrating.

My first reaction was that the floor is too similar to the hardwood. I also don't like the taupey purple undertone, which is very common in gray tiles. I would not use a stone-look at all with those more cottage-y tub tiles as the styles are very different. I think it's too much pattern for a modest bathroom so I'd look for a very subtle pattern, maybe something in a linen look if you want to use a large format tile.

Kohler has the Ceramic Impressions line of countertop with integrated sink, but they come in standard sizes. Here is an old thread of mine with photos.

[https://www.houzz.com/discussions/what-do-you-think-of-the-kohler-impressions-ceramic-countertops-sink-dsvw-vd~3433846[(https://www.houzz.com/discussions/what-do-you-think-of-the-kohler-impressions-ceramic-countertops-sink-dsvw-vd~3433846)