Thanks everyone for their comments! Update: husband has 'over-ruled' on this one. Forgot to mention that the vanity top will be Night Sky Ceaserstone (ie black). So he wants the dark grout. We will definitely be using mould inhibitor and ensuring everything is well sealed. It's all his fault if it looks horrible ;) Will post photos once fully complete. (I am now a little scared...)

What a beautiful tile! Rule of the tump - always match the grout to the tile. Specially with such a strong patterned tile, otherwise the grout also becomes a clashing pattern. I'd pick light grey to avoid pure white cleaning issues. I have these type of tiles in my patio, you will have to be seal them with penetrating sealer after installation, but other then that you will love them forever

It's a lovely tile! I'd go light-mid grey (basically like the picture!), if you go dark then it's pointless having the interesting hexagonal shape and the veins will look a bit odd stopping and starting, and if you go too light it will compete with the veins in the marble and look to busy.

I agree on the soft grey for the floor tiles. I think anything else would detract from the pattern of the lovely floor tiles. I can see why you asked for help with the wall grout. I can think of 4 different reasons why I'd choose different colours! Maybe look at some similar tiles (real life or photos) with similar coloured tiles and different grout colours.