You have a very clean look going, I'm personally more a fan of matching the grout to the tile instead of using a contrasting grout, so I would use white grout in your kitchen. But once the tile is up, before it is grouted, you will be able to get a decent idea on whether you might prefer a darker grout instead since ungrouted white tile resembles white tile with a darker grout.

I used a very light grey with our white tiles.

We used a light gray with our light gray quartz counters

I never like white grout it never stays white so i would do a platinum color

What a beautiful kitchen! I'm very impressed!

I agree about white grout not staying white. I also like the modern look of contrasting white subway tiles with a grey grout. BUT! You have a grey countertop, so it will make the grout pop a lot more than a white one would.

As long as you keep the grout a very light grey, I think it will look great: