You can stain the grout. I'd do white.

Not white thats for sure unless you just love scrubbing on your hands and knees a few times a year.

Poly grout renew. Apply with a children's toothbrush.

I would go gray. I put white grout on a floor with similar tile and yes, you will be scrubbing it all of the time to distraction.

I would also go with gray- much better at hiding dirt than white.

If you use white, it won't stay white no matter how often you clean it, so you might as well go with the gray.

Gray!

Another vote for the deeper gray. My similar tile is only 2 years old and I've already had to have the pale (almost white) pearl gray grout professionally cleaned -- and it still doesn't look like right.

Definitely gray.

You can't go wrong either way: Your white will soon look grey.