grey glass would look good

I would suggest possibly using a backsplash that combines glass and slate or stone. Find one that brings out the gray in the countertop in the glass and ties in the stone of the fireplace. Best of both worlds. An example:

I always go with a simple approach, I recommend a full height backsplash made of the same material as the countertop.

I would pick something in the gray or silver family since it will be right next to counter. The other colors in the next room (black, brown, tans) are neutrals so I think it would look great.