inability to distinguish one or more of the three colours red, green, and blue. Most people with colour-vision problems have a weak colour-sensing system rather than a frank loss of colour sensation. In the retina (the light-sensitive layer of tissue that lines the back and sides of the eyeball), humans have three types of cones (the visual cells that function in the perception of colour). One type absorbs light best in wavelengths of blue-violet and another in the wavelengths of green. The third type is most sensitive to longer wavelengths—more sensitive to red.

Colour-blind persons may be blind to one, two, or all of the colours red, green, and blue. (Blindness to red is called protanopia; to green, deuteranopia; and to blue, tritanopia.) Red-blind persons are ordinarily unable to distinguish between red and green, while blue-blind persons cannot distinguish between blue and yellow. Green-blind persons are unable to see the green part of the spectrum.

Hereditary red-green colour blindness, which affects about 20 times as many males as females, is a sex-linked recessive characteristic. A woman must inherit the trait from both parents to be red-green colour-blind. A red-green colour-blind man and a woman of normal colour vision have daughters who have normal colour vision but are carriers of the trait—that is, the daughters may have red-green colour-blind sons and daughters who are carriers. The sons of a red-green colour-blind man and a woman with normal vision who themselves have normal vision are unable to pass the red-green colour-blind trait on to offspring. The son of a normal man and a carrier woman may be red-green colour-blind, and the daughter of such a union may be a carrier. Thus, red-green colour blindness tends to skip generations.

Acquired colour blindness is usually of the blue-yellow type and can be due to retinal diseases, glaucoma, or optic nerve diseases. Total colour blindness (achromatopsia) is an extremely rare congenital affliction that is typically associated with poor vision, nystagmus (rapid, uncontrollable eye movements), and light sensitivity.