The term colour blindness, or colour vision deficiency, refers to all deficiencies where one cannot see certain colours. There are several different kinds of colour blindness, such as those who cannot see different shades of the colour red, green or purple. Red-green colour blindness is the most common, followed by blue-yellow.
The most unusual and rare form is called monochromacy, where the world is seen in black and white. Only about one out of 30,000 people are affected. They can distinguish light and dark, and shades of grey.
Males are more likely to be colour blind than females because the genes responsible for most kinds of colour blindness are on the X chromosome. Females have two X chromosomes, so one can usually make up for the deficiency in the other. However females can be carriers, and since males have only one X chromosome, they are then more affected.
The first scientific paper written on colour blindness was in 1798 by chemist John Dalton, who had realised his own colour blindness. Ever wondered how colour blind people see the world? Several websites such as colourblindness.com exist, where a photo can be uploaded. The website then shows you how a person with a certain kind of colour blindness would see that image.