I think any of your options work. Personally with such a lovely stone countertop, I would go with white grout to keep the focus on the counters.

white..but whithin whites there are usually a couple...don't pick the whitest white....look at off-white. Very light gray would be fine too.

Agree w/ Lilly'smom. I might have those same exact subway tiles and did medium grey because I wanted the tiles to stand out.

DF who used to tile for a living, advised me to buy a couple small bags of grout in the colors I was considering and dry grout the tile (obviously while the tile was lying on a flat surafce). That was $$ well spent, I knew immediately which color I wanted.

Just note that contrasting grout will highlight any tiling of grouting imperfection you have and drive you insane. Your tile looks very well done and this may not be an issue for the tiling, but then even the grouting needs to be perfect - the exact same amount removed everywhere for smooth consistent grout lines. It's a great look with contrasting grout - very current -but not all that easy to master and MAY end up being a little trendy.

I'd go with a light grey. And punch up the space with colorful accessories (as much as I think that the BS would look awesome with that island having more contrast!!!!)

Honestly, I think it's a personal preference thing. I've put in subways in various rooms for over 10 years and I always did contrasting grout b/c I like to be able to see that distinction and highlight the pattern. To me, white grout looks blah and artificial somehow. But I've had tilers who have argued vehemently with me that I'm "doing it wrong" when I ask for contrasting grout. I like a really dark contrast, but like jannicone says, the darker the grout the more perfect the tiling job has to be. I've always done a light or medium tone contrast for that reason.

Looking at your pics, I think contrasting grout would tie in beautifully with the darker lines of your counter. I think it would also help to break up the all white of the uppers with the tile. Nothing drastic, just a light to med. grey - but that's just MHO.

In your space a light gray grout would be perfect! I like white on white more in bathroom spaces.

Mostly I like matching grout (white), but in your case I think the light gray may be a better choice. I'm looking at the very white cabs and the white and gray granite and thinking the light gray might tie them together nicely.

I am not a fan of white grout with white tile because of differing tones of white grout. I would go with a very light grey (almost white) if you are a fan of white grout or a light grey if you like some contrast.

Timble, here is a pic of my white subway tile with a light gray grout.