In ancient Egypt, color was an integral part of the substance and being of everything in life. The color of something was a clue to the substance or heart of the matter. When it was said that one could not know the color of the gods, it meant that they themselves were unknowable, and could never be completely understood. In art, colors were clues to the nature of the beings depicted in the work. For instance, when Amon was portrayed with blue skin, it alluded to his cosmic aspect. Osiris' green skin was a reference to his power over vegetation and to his own resurrection.