question: how people can have eye colors such as blue and green but not orange or purple?
comment: Eye color is determined by how much or how little melanin there is in the iris. More melanin leads to the darker shades and green irises, and having little to no melanin gives us the blue and grey irises. Since melanin is what gives us the pigment in our iris, the color limitations of melanin is why there aren't any more different colors of eyes.
passages: There are many other possible reasons for having two different-colored eyes. For example, the film actor Lee Van Cleef was born with one blue eye and one green eye, a trait that reportedly was common in his family, suggesting that it was a genetic trait. This anomaly, which film producers thought would be disturbing to film audiences, was "corrected" by having Van Cleef wear brown contact lenses. David Bowie, on the other hand, had the appearance of different eye colors due to an injury that caused one pupil to be permanently dilated.
   The eyes of people with severe forms of albinism may appear red under certain lighting conditions owing to the extremely low quantities of melanin, allowing the blood vessels to show through. In addition, flash photography can sometimes cause a "red-eye effect", in which the very bright light from a flash reflects off the retina, which is abundantly vascular, causing the pupil to appear red in the photograph. Although the deep blue eyes of some people such as Elizabeth Taylor can appear violet at certain times, "true" violet-colored eyes occur only due to albinism.
  
  Section::::Medical implications.
   List of people with heterochromia
  
  This is a list of notable people who have been documented as having heterochromia iridis, a condition when the irises have different colours.
  
  Section::::Actors.
  
  BULLET::::- Gracie Allen
  
  BULLET::::- Dan Aykroyd
  
  BULLET::::- Àstrid Bergès-Frisbey
  
  BULLET::::- Elizabeth Berkley
  
  BULLET::::- Kate Bosworth
  
  BULLET::::- Henry Cavill
  
  BULLET::::- Garrett Clayton
  
  BULLET::::- Benedict Cumberbatch
  
  BULLET::::- Robert Downey, Jr.
  
  BULLET::::- Norma Eberhardt
  
  BULLET::::- Alice Eve
  
  BULLET::::- James Forde
  
  BULLET::::- Alyson Hannigan
  
  BULLET::::- Josh Henderson
  
  BULLET::::- Liam James
  
  BULLET::::- Mila Kunis
  
  BULLET::::- Jonathan Rhys Meyers
  
  BULLET::::- Colleen Moore
  
  BULLET::::- Simon Pegg
  
  BULLET::::- Joe Pesci
  
  BULLET::::- Jane Seymour
  
  BULLET::::- Dominic Sherwood
  
  BULLET::::- Kiefer Sutherland
   Eye color in non-human animals is regulated differently. For example, instead of blue as in humans, autosomal recessive eye color in the skink species "Corucia zebrata" is black, and the autosomal dominant color is yellow-green.
  
  As the perception of color depends on viewing conditions (e.g., the amount and kind of illumination, as well as the hue of the surrounding environment), so does the perception of eye color.
  
  Section::::Changes in eye color.
   The iris is usually strongly pigmented, with the color typically ranging between brown, hazel, green, gray, or blue. Occasionally, the color of the iris is due to a lack of pigmentation, as in the pinkish-white of oculo-cutaneous albinism, or to obscuration of its pigment by blood vessels, as in the red of an abnormally vascularised iris. Despite the wide range of colors, the only pigment that contributes substantially to normal human iris color is the dark pigment melanin. The quantity of melanin pigment in the iris is one factor in determining the phenotypic eye color of a person. Structurally, this huge molecule is only slightly different from its equivalent found in skin and hair. Iris color is due to variable amounts of eumelanin (brown/black melanins) and pheomelanin (red/yellow melanins) produced by melanocytes. More of the former is found in brown-eyed people and of the latter in blue and green-eyed people.
   Section::::In mammals.
  
  The only pigments that mammals produce are melanins. For a mammal to be unable to chemically manufacture melanin renders it completely pigmentless. This condition is more commonly called albinism. Amelanistic mammals have white hair, pink skin, and eyes that have a pink, red, or violet appearance. Reddish eyes are due to the lack of pigment in the iris pigment epithelium. When the stroma is unpigmented but the iris pigment epithelium is not, mammalian eyes appear blue. Melanin in the pigment epithelium is critical for visual acuity and contrast.
   Most new-born babies who have European ancestry have light-colored eyes. As the child develops, melanocytes (cells found within the iris of human eyes, as well as skin and hair follicles) slowly begin to produce melanin. Because melanocyte cells continually produce pigment, in theory eye color can be changed. Adult eye color is usually established between 3 and 6 months of age, though this can be later. Observing the iris of an infant from the side using only transmitted light with no reflection from the back of the iris, it is possible to detect the presence or absence of low levels of melanin. An iris that appears blue under this method of observation is more likely to remain blue as the infant ages. An iris that appears golden contains some melanin even at this early age and is likely to turn from blue to green or brown as the infant ages.
   Eye color is determined primarily by the concentration and distribution of melanin within the iris tissues. Although the processes determining eye color are not fully understood, it is known that inherited eye color is determined by multiple genes. Environmental or acquired factors can alter these inherited traits.
  
  The human iris can be seen in a number of various colors. There are three true colors in human eyes that determine the outward appearance: brown, yellow, and grey. The amount of each color an individual has determines the appearance of the eye color.
   The gene "OCA2" (), when in a variant form, causes the pink eye color and hypopigmentation common in human albinism. (The name of the gene is derived from the disorder it causes, oculocutaneous albinism type II.) Different SNPs within "OCA2" are strongly associated with blue and green eyes as well as variations in freckling, mole counts, hair and skin tone. The polymorphisms may be in an "OCA2" regulatory sequence, where they may influence the expression of the gene product, which in turn affects pigmentation. A specific mutation within the "HERC2" gene, a gene that regulates "OCA2" expression, is partly responsible for blue eyes. Other genes implicated in eye color variation are SLC24A4 and TYR. A 2010 study on eye color variation into hue and saturation values using high-resolution digital full-eye photographs found three new loci for a total of ten genes, and now about 50% of eye colour variation can be explained.
   Variation in cone pigment genes is wide-spread in most human populations, but the most prevalent and pronounced tetrachromacy would derive from female carriers of major red/green pigment anomalies, usually classed as forms of "color blindness" (protanomaly or deuteranomaly). The biological basis for this phenomenon is X-inactivation of heterozygotic alleles for retinal pigment genes, which is the same mechanism that gives the majority of female new-world monkeys trichromatic vision.
   Section::::Eye color.:Genetic and physical factors determining iris color.
  
  Iris color is a highly complex phenomenon consisting of the combined effects of texture, pigmentation, fibrous tissue and blood vessels within the iris stroma, which together make up an individual's epigenetic constitution in this context. A person's "eye color" is actually the color of one's iris, the cornea being transparent and the white sclera entirely outside the area of interest.
   Normally, there is a thick layer of melanin on the back of the iris. Even people with the lightest blue eyes, with no melanin on the front of the iris at all, have dark brown coloration on the back of it, to prevent light from scattering around inside the eye. In those with milder forms of albinism, the color of the iris is typically blue but can vary from blue to brown. In severe forms of albinism, there is no pigment on the back of the iris, and light from inside the eye can pass through the iris to the front. In these cases, the only color seen is the red from the hemoglobin of the blood in the capillaries of the iris. Such albinos have pink eyes, as do albino rabbits, mice, or any other animal with a total lack of melanin. Transillumination defects can almost always be observed during an eye examination due to lack of iridial pigmentation. The ocular albino also lacks normal amounts of melanin in the retina as well, which allows more light than normal to reflect off the retina and out of the eye. Because of this, the pupillary reflex is much more pronounced in albino individuals, and this can emphasize the red eye effect in photographs.
   Homozygous cream dilutes, sometimes called double-dilutes, always have light blue eyes to match their pale, cream-colored coats. Heterozygous or single-dilute creams, such as palominos and buckskins, often have light brown eyes. The eyes of horses with the Champagne gene are typically greenish shades: aqua at birth, darkening to hazel with maturity.
  
  As in humans, much of the genetics and etiology behind eye color are not yet fully understood.
  
  Section::::The anatomy of the equine eye.:The adnexa.
   Section::::Genetic determination.
   Many species, such as all marine mammals, the owl monkey and the Australian sea lion (pictured at right) are monochromats under normal conditions. In humans, absence of color discrimination or poor color discrimination is one among several other symptoms of severe inherited or acquired diseases, as for example inherited achromatopsia, acquired achromatopsia or inherited blue cone monochromacy.
  
  Section::::Humans.
   The brightly colored eyes of many bird species result from the presence of other pigments, such as pteridines, purines, and carotenoids. Humans and other animals have many phenotypic variations in eye color. The genetics of eye color are complicated, and color is determined by multiple genes. So far, as many as 15 genes have been associated with eye color inheritance. Some of the eye-color genes include "OCA2" and "HERC2". The earlier belief that blue eye color is a simple recessive trait has been shown to be incorrect. The genetics of eye color are so complex that almost any parent-child combination of eye colors can occur. However, OCA2 gene polymorphism, close to proximal 5′ regulatory region, explains most human eye-color variation.
   It is fairly common for mammalian eyes to change color in the first years of life. This happens, with human infants and kittens being some well-known examples, because the eyes of the baby, just like the rest of its body, are still developing. This change can be as simple as blue to brown, or can involve multiple color changes in which neither the child's parents nor his/her doctors know when the changes will stop and what the final eye color will be.
   While most humans are "trichromatic" (having three types of color receptors), many animals, known as "[[Tetrachromacy|tetrachromats]]", have four types. These include some species of [[spider]]s, most [[marsupial]]s, [[bird]]s, [[reptile]]s, and many species of [[fish]]. Other species are sensitive to only two axes of color or do not perceive color at all; these are called "dichromats" and "monochromats" respectively. A distinction is made between "retinal tetrachromacy" (having four pigments in cone cells in the retina, compared to three in trichromats) and "functional tetrachromacy" (having the ability to make enhanced color discriminations based on that retinal difference). As many as half of all women are retinal tetrachromats. The phenomenon arises when an individual receives two slightly different copies of the gene for either the medium- or long-wavelength cones, which are carried on the [[X chromosome]]. To have two different genes, a person must have two X chromosomes, which is why the phenomenon only occurs in women. There is one scholarly report that confirms the existence of a functional tetrachromat.
   A further difference between albinism and leucism is in eye colour. Due to the lack of melanin production in both the retinal pigmented epithelium (RPE) and iris, those affected by albinism typically have red eyes due to the underlying blood vessels showing through. In contrast, most leucistic animals have normally coloured eyes. This is because the melanocytes of the RPE are not derived from the neural crest, instead an outpouching of the neural tube generates the optic cup which, in turn, forms the retina. As these cells are from an independent developmental origin, they are typically unaffected by the genetic cause of leucism.
   Section::::Classification of color.
  
  Iris color can provide a large amount of information about a person, and a classification of colors may be useful in documenting pathological changes or determining how a person may respond to ocular pharmaceuticals. Classification systems have ranged from a basic light or dark description to detailed gradings employing photographic standards for comparison. Others have attempted to set objective standards of color comparison.
   White babies are usually born blue-eyed since there is no pigment in the stroma, and their eyes appear blue due to scattering and selective absorption from the posterior epithelium. If melanin is deposited substantially, there will be brown or black color, if not, they will remain blue or gray.
  
  All the contributing factors towards eye color and its variation are not fully understood. Autosomal recessive/dominant traits in iris color are inherent in other species but coloration can follow a different pattern.
  
  Section::::Eye color.:Genetic and physical factors determining iris color.:Amber eyes.
   In humans, the pigmentation of the iris varies from light brown to black, depending on the concentration of melanin in the iris pigment epithelium (located on the back of the iris), the melanin content within the iris stroma (located at the front of the iris), and the cellular density of the stroma. The appearance of blue and green, as well as hazel eyes, results from the Tyndall scattering of light in the stroma, a phenomenon similar to that which accounts for the blueness of the sky called Rayleigh scattering. Neither blue nor green pigments are ever present in the human iris or ocular fluid. Eye color is thus an instance of structural color and varies depending on the lighting conditions, especially for lighter-colored eyes.
   A chimera can have two different colored eyes just like any two siblings can—because each cell has different eye color genes. A mosaic can have two different colored eyes if the DNA difference happens to be in an eye-color gene.
   Although the eye color is described as having depth "even greater even than that seen in a Siamese (cat)" it, in fact, does not. The blue color seen in cats is due to the same physical phenomenon, the absence of melanin in the iris. The cause of the absence of pigment differs between Siamese, white cats, and Ojos but the end result is pigment loss and blue eyes. The depth of color of the blue eyes is due to currently unknown polygenetic variations. The depth of eye color can be selected for in breeding programs and current show Siamese have extremely deep eye color.
   Furthermore, color is distinguishable due to the different iodopsins of cone cells; there are three different kinds, in normal human vision, which is why we need three different primary colors to make a color space.
  
raw_presuppositions: The range of eye colors should be infinate.
raw_corrections: The range of eye colors is limited by the color limitations of melanin.