The gray color was standard for everything that was sticking out above the water line. White color is high contrast, and would be seen easily by aircraft or passing ships. I think the color of the horizon has something to do with it. There were instances where part or whole of the conning tower was painted light gray- almost white, but it was reserved for boats operating near the ice cups of the north sea.
I have close up picture of the periscopes, and even tho they are black, and white-the scopes are deffinitely gray.
Regarding the gulley, i dont know exactly, but i thought they were situated below the control room floor.