Color blindness

Decreased ability to see color or color differences

Color blindness or color vision deficiency (CVD) is the decreased ability to see color, differences in color, or distinguish shades of color. The severity of color blindness ranges from mostly unnoticeable to full absence of color perception. Color blindness is usually a sex-linked inherited problem or variation in the functionality of one or more of the three classes of cone cells in the retina, which mediate color vision.

From Wikipedia, under CC BY-SA. More on occurri.