A fairly light gray such as Custom Building Products #165 Delorean Gray. See installations below. One seems to have a slightly lighter grout and the other a slightly darker grout. Your installer could prepare a couple of samples for you, but would probably charge extra to do so.

I just did a kitchen backsplash with that exact tile, and we used Mapei's Gray. It's a medium to darker color gray:

Here's a closer look. (sorry for the double post-- just getting used to this forum!)

a grey, white or tan colour would be good. Take into consideration your paint colour.