On Vision and Colors are, however, nonessential properties that differ according to the type of light-refracting substance that is used. The top of the upper image is violet. Below the violet is blue. The bottom of the lower image is orange. Above the orange is yellow. In this way, along with the white disk and the black background, four prismatic colors appear: violet, blue, yellow, and orange. This is in disagreement with Newton's claim that there are seven prismatic colors. As the upper image overlaps black, it is seen as violet. Where it overlaps white, it is seen as blue. As the lower image
Complementary colors effect that colors have upon each other had been noted since antiquity. In his essay "On Colors", Aristotle observed that "when light falls upon another color, then, as a result of this new combination, it takes on another nuance of color." Saint Thomas Aquinas had written that purple looked different next to white than it did next to black, and that gold looked more striking against blue than it did against white; the Italian Renaissance architect and writer Leon Battista Alberti observed that there was harmony ("coniugatio" in Latin, and "amicizia" in Italian) between certain colors, such as red–green and