Color blindness occurs when certain color-sensing cones in the retina fail. The retina in the human eye is covered, with varying densities, with light sensors called rods and cones; rods detect the level of brightness in the light striking them, while cones detect color. Certain cones detect red light, others blue, and others green. If one set of these cones fails to detect light properly, however -- say the red cones fail, so that purple and blue colors are indistinguishable -- then color blindness occurs.