These tiles are textures. If you have a display with a broken contrast/brightness ratio, like most TFT screens, you won't notice anything else than a 100% white square. Open these things on a CRT screen and you'll see there's a texture in it. Zooming on my TFT screen shows me there's some structure in it, but that's all I see on my left display (I don't see the differences between a light shade of gray and white on that screen either).