Transparency means something one can look through with clarity, like a sheet of glass or plastic. So what we are interested in is really translucency—the state or condition of being translucent, or partially transparent. But translucency is essentially an optical illusion, or trick of the human eye. When a light color contrasts with a dark color, and the light color is apparently “above” the darker color, then the human eye is trained to perceive degrees of translucency.