Pewter, Sahara Beige- Gray (may be a little on dark side)

Do you want the backsplash to stand out or "go away"? Dark grout if you want it to be eye-catching, color match the tile grout if you want it to "go away". The kitchen cabinets are very modern/contemporary, so it's a matter of personal taste. Think about the overall intention.

Everything is so white, so I'm thinking a little contrast!

For contrast, I would stick to a similar color grout as the color of your floor tiles. Otherwise, I think it may look "willy nilly"! You can always bring in other colors, but leaving the non-changable items basic, you won't get tired of it.

For kicks I like to toss out the bright lipstick red grout color when people ask for my opinion.
Considering you have a dark floor and everything else is stark white, I think you should go with a lighter gray color for your grout. Gives a little contrast but won't stick out like a sore thumb.