There are two kinds of red-green color blindness: protanopia or deuteranopia. Deuteranopia is the most common form of color blindness; between five and ten percent of males suffer from it. It is called Daltonism because John Dalton discovered it. Those affected have trouble telling the difference between red and green. As seen by protanopes (people suffering from protanopia), red is darkened. Most often, this is because they lack the receptors for long (protanopia) or medium (deuteranopia)-length light waves or because these receptors have changed their sensitivity.