Totally agree with simple subway tile due to the pattern in the granite. I would pick something that blends with the lightest color in the granite, so your thinking sounds good to me.

Yes a subway that matches the lightest color in the countertop would be a huge upgrade for your kitchen. Nice job on the paint color, it looks great.

there is such a large amount of very dark and then cool whites......you could consider a very pale green to provide a softer hue which will give some BALANCE. Think: when a silver haired mature woman puts on a black dress or outfit...a soft silk scarf w some color assists with the overall. Do consider something other than white for the tiles.....