Color blindness, a color vision deficiency, is the inability to perceive differences between some of the colors that others can distinguish. The most frequent forms of human color blindness involve difficulties in discriminating reds, yellows, and greens from one another. They are collectively referred to as "red-green color blindness", Other forms of color blindness are much more rare. They include problems in discriminating blues from yellows, and the rarest forms of all, complete color blindness or where one cannot distinguish any color from grey.