While on the outside of the Castle, particularly at the start of the path to the Western Reflector, some of the tiles are almost blindingly white. This must be the original appearance of all the exterior tiles, at least on the front of the Castle in the Mists.
This may have been taken from the Pyramids of Egypt. Originally they were completed covered in a shimmering white stone. To quote Wikipedia, Most pyramids had a polished, highly reflective white limestone surface, in order to give them a shining appearance when viewed from a distance. Imagine what the Castle must have looked like new.