[2] The most common form is caused by a genetic condition called congenital red–green color blindness (including protan and deutan types), which affects up to 1 in 12 males (8%) and 1 in 200 females (0.5%).

Color blindness can also result from physical or chemical damage to the eye, the optic nerve, parts of the brain, or from medication toxicity.

[2] Depending on the jurisdiction, the color blind are ineligible for certain careers,[1] such as aircraft pilots, train drivers, police officers, firefighters, and members of the armed forces.

One such case is a traffic light in Tipperary Hill in Syracuse, New York, which is upside-down (green–amber–red top to bottom) due to the sentiments of its Irish American community.

In the eastern provinces of Canada, traffic lights are sometimes differentiated by shape in addition to color: square for red, diamond for yellow, and circle for green (see image).

The 20th century expressionist painter Clifton Pugh, three-time winner of Australia's Archibald Prize, on biographical, gene inheritance and other grounds has been identified as a person with protanopia.

[17] Dichromats tend to learn to use texture and shape clues and so may be able to penetrate camouflage that has been designed to deceive individuals with normal color vision.

A common task for data visualization is to represent a color scale, or sequential colormap, often in the form of a heat map or choropleth.

Several scales are designed with special consideration for the color blind and are widespread in academia, including Cividis,[25] Viridis[25] and Parula.

[27] Partial color blindness includes dichromacy and anomalous trichromacy, but is often clinically defined as mild, moderate or strong.

The terms protanopia, deuteranopia, and tritanopia come from Greek, and respectively mean "inability to see (anopia) with the first (prot-), second (deuter-), or third (trit-) [cone]".

Mechanisms for color blindness are related to the functionality of cone cells, and often to the expression of photopsins, the photopigments that 'catch' photons and thereby convert light into chemical signals.

The most common forms of color blindness are associated with the Photopsin genes, but the mapping of the human genome has shown there are many causative mutations that do not directly affect the opsins.

[45] Several inherited diseases are known to cause color blindness, including achromatopsia, cone dystrophy, Leber's congenital amaurosis and retinitis pigmentosa.

Color blindness may also present itself as a symptom of degenerative diseases of the eye, such as cataract and age-related macular degeneration, and as part of the retinal damage caused by diabetes.

Actual physiological examples usually affect the blue–yellow opponent channel and are named Cyanopsia and Xanthopsia, and are most typically an effect of yellowing or removal of the lens.

[1] Some tests are clinical in nature, designed to be fast, simple, and effective at identifying broad categories of color blindness.

However, it can also be used to sequence the L- and M-Opsins on the X-chromosome, since the most common alleles of these two genes are known and have even been related to exact spectral sensitivities and peak wavelengths.

[68][69] Interestingly, even Dalton's first paper already arrived upon this 8% number:[70] ...it is remarkable that, out of 25 pupils I once had, to whom I explained this subject, 2 were found to agree with me...During the 17th and 18th century, several philosophers hypothesized that not all individuals perceived colors in the same way:[71] ...there is no reason to suppose a perfect resemblance in the disposition of the Optic Nerve in all Men, since there is an infinite variety in every thing in Nature, and chiefly in those that are Material, 'tis therefore very probable that all Men see not the same Colours in the same Objects.In the power of conceiving colors, too, there are striking differences among individuals: and, indeed, I am inclined to suspect, that, in the greater number of instances, the supposed defects of sight in this respect ought to be ascribed rather to a defect in the power of conception.Gordon Lynn Walls claims[74] that the first well-circulated case study of color blindness was published in a 1777 letter from Joseph Huddart to Joseph Priestley, which described "Harris the Shoemaker" and several of his brothers with what would later be described as protanopia.

[74] The phenomenon only came to be scientifically studied in 1794, when English chemist John Dalton gave the first account of color blindness in a paper to the Manchester Literary and Philosophical Society, which was published in 1798 as Extraordinary Facts relating to the Vision of Colours: With Observations.

A Brazilian court ruled that the color blind are protected by the Inter-American Convention on the Elimination of All Forms of Discrimination against Person with Disabilities.

[84][85][86] At trial, it was decided that the carriers of color blindness have a right of access to wider knowledge, or the full enjoyment of their human condition.

[80] ...I consider that to [Holmgren] above all others do we owe the present and future control of color-blindness on land and sea, by which life and property are safer, and the risks of travelling less.Color vision is important for occupations using telephone or computer networking cabling, as the individual wires inside the cables are color-coded using green, orange, brown, blue and white colors.

[87] Electronic wiring, transformers, resistors, and capacitors are color-coded as well, using black, brown, red, orange, yellow, green, blue, violet, gray, white, silver, and gold.

Professional football players Thomas Delaney and Fabio Carvalho have discussed the difficulties when color clashes occur, and research undertaken by FIFA has shown that enjoyment and player progression can be hampered by issues distinguishing the difference between the pitch and training objects or field markings.

[89] Snooker World Champions Mark Williams and Peter Ebdon sometimes need to ask the referee for help distinguishing between the red and brown balls due to their color blindness.

Protans are further disadvantaged due to the darkened perception of reds, which can make it more difficult to quickly recognize brake lights.

Some examples include color-gun signaling of aircraft that have lost radio communication, color-coded glide-path indications on runways, and the like.

If an applicant fails the standard tests, they will receive a restriction on their medical certificate that states: "Not valid for night flying or by color signal control".

[101] Research published in 2009 carried out by the City University of London's Applied Vision Research Centre, sponsored by the UK's Civil Aviation Authority and the U.S. Federal Aviation Administration, has established a more accurate assessment of color deficiencies in pilot applicants' red/green and yellow–blue color range which could lead to a 35% reduction in the number of prospective pilots who fail to meet the minimum medical threshold.