I think a medium gray would look fine. Be sure to caulk the area where the tile meets the backsplash. Use a color-matching silicone caulk to accommodate the microscopic movement that is inevitable there.

Depends on what aesthetic you're going for - if you want a really classic look dark grout works. What does the rest of your house look like? Is it old? Modern? Do you keep a lot of stuff on your counter? Dark grout might make it look too busy in that case...

If you're looking for the backsplash to be the star, I say go with dark. If you're just looking for a pretty, light reflective backdrop for your pretty things, I'd stay with a matching grout. My two cents at least.

I just had white subways installed in my shower/tub alcove. I chose the next to lightest gray grout and I wish I had chosen the lightest. I didn't want white grout, but just the merest suggestion of lines. But it's fine. You do stop seeing the lines.

Your granite looks beige/tan/brown-ish. Not sure I would use gray grout, unless there's gray elsewhere in your kitchen.