Some sort of dark grey by the looks of it. Just a word of warning- darker grout will shoe EVERY imperfection in the tile setting. You can see it in the example photo, especially on the top of the niche. If you want the dark grout and that contrast you need a REALLY good tile setter. A light grey would still give you the contrast while being more forgiving.

Notice how the running bond is off especially in the row of tiles above the niche? That would bother me no end. I mention this because you say that you want your bathroom exactly like this. I would also use a lighter gray.

A lighter gray is ok too - specifically what gray grout do you think? Dalorean?

In the photo, the tile layout is all wonky and wrong, so please don’t give this photo to your tile pro!! The layout shifts above the niche, and in the corner, the tile slivers get smaller and smaller as you move your eye from top to bottom. The top of the niche doesn’t line up with the tile layout, and there’s an unsightly “double line” on either side of the niche frame; I can’t even tell what that is and why it’s there.

Grout: lighter than this. If the owner of this bathroom had used a lighter grout color, these bone-headed layout problems would not be as noticeable. Perfect layout and installation with a highly experienced and NTCA certified tile pro? Okay, go ahead and use a dark grout. Anything less than that? Mapei “Frost.”