Of blackness, redness, yellowness, and other things, some of them are a permanent color in the place, and some of them are a color in the eyes of the beholder, but they are not the same in color, such as the blackness of white mountains in the distance. If you come to them, you see them as white, and you judged them to be black, and you were wrong in that ruling, and it is correct in the appearance of blackness. It is a disaster, and the method of that is unknown. In this case, the blue of the sky is only for the eye to see, even if it is in itself a color that contradicts the blue.