You might want to go with something that ties in with the granite you have. Dark grey or a charcoal color.

Thank you. What do you mean mirror? I havent seen this before do you have any pics?

I was thinking grey. But mirror may help kitchen look bigger. I wish I had pics

I worked in a hotel years ago with a small kitchen in the Concierge Lounge. They had a backsplash installed that was mirror (a glass company could install. ) You might look at mirrored tiles as a less expensive route than custom. It opened it up, and it had no windows either.

Type in mirror + backsplash for photos in Houzz

I would go with a backpainted glass. It will reflect the light. If there is a green in your granite, or a light gray.

I would keep it light so as not to make the kitchen smaller. Either a solid mirror, not tiles, for a clean and reflective look. Or as above, backpainted glass, again in a light colour. Good luck.