Colour-blindness

Incapacity of discerning one colour from another. The term was introduced by Sir David Brewster. It is of three sorts: (1) inability to discern any colours, so that everything is either black or white, shade or light; (2) inability to distinguish between primary colours, as red, blue, and yellow; or secondary colours, as green, purple, and orange; and (3) inability to distinguish between such composite colours as browns, greys, and neutral tints. Except in this one respect, the colour-blind may have excellent vision.