I'd stick to your grey and white theme. Sample out some subway tiles of different finishes in both colors. Use some color for accessories and flowers, right now the spaces are reading a bit drab. That's not an insult because I think the space is clearly beautiful, it just needs to be livened up a tad.

Dal tiles Chrystal shores(Diamond Delta CS93) 2 x 1 Brick Might be a great mix to use for your Kitchen. I think it would be a great way bring everything together. You could use the coordinating white subway tile and use this as just a boarder also. Good luck I cant wait to see what you choose.

My first thought when looking at your kitchen is a white glass subway tile. They also make these beveled to add more dimension.

Love this classic marble style..would be beautiful in your kitchen.