the simplest way is to order samples and bring them home to look at in your light. many many different shades of grays and whites. you could do a glass in a glossy or a frosted. or a ceramic in a polished or matte finish.

order some sample online (tilebar and glass tile oasis has 5 for $5)

and of course you know what white subway looks like, because everyone on the planet is doing white subway tile.