It’s the context of shape that tells the eye to interpret a thing as flesh, stone, glass or steel. Color has very little to do with it. The color of his helmet may seem to make it look like steel, but the same colors on a coffee mug, would look ceramic, on a rhinoceros would look like skin, on the surface of a lake would look like water. It's the shape of the thing, the intensity of the light and dark, the hard sharp edges and soft smooth curves, and the fact that it's on his head. That's what tells you what it's made of, not the color. So when you ask yourself "what color should this thing be?" The answer is "just about any color so long as it otherwise looks convincing."