Color
Colors are seldom seen in isolation. Much of the richness and variety of color is due to the complex ways in which colors react to one another. The apparent value and intensity of a color is especially dependent upon the other colors that happen to surround it. In the example notice how the changing value and hue of the circles affect the readability. Because of the differences in human vision, not all individuals will be able to decipher the change in hue at the same time.