question: Why do our eyes have color? Is there a role that the iris having a color plays?
comment: If the iris wasn’t coloured, the amount of light entering our pupil would actually be increased due to the thinness of the membrane. Everyone technically has blue eyes (afaik) and then additional melanin deposits on top, adding to the brown colour. The darker it is, the more light it absorbs and the less light enters the eye. This acts in addition to the standard dilation and contraction.
passages: 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.
   Section::::Mate selection and traits that have been linked to iris color.
  
  Section::::Mate selection and traits that have been linked to iris color.:Selection for rare iris colors.
   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.
   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.
   Iris (anatomy)
  
  In humans and most mammals and birds, the iris (plural: irides or irises) is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.
  
  Section::::Structure.
  
  The iris consists of two layers: the front pigmented layer known as a stroma and, beneath the stroma, pigmented epithelial cells.
   BULLET::::- Anterior pigment epithelium
  
  BULLET::::- Posterior pigment epithelium
  
  Section::::Structure.:Development.
  
  The stroma and the anterior border layer of the iris are derived from the neural crest, and behind the stroma of the iris, the sphincter pupillae and dilator pupillae muscles as well as the iris epithelium develop from optic cup neuroectoderm.
  
  Section::::Eye color.
   Section::::Eye color.:Different colors in the two eyes.
   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.
   Melanin is yellowish-brown to dark brown in the stromal pigment cells, and black in the iris pigment epithelium, which lies in a thin but very opaque layer across the back of the iris. Most human irises also show a condensation of the brownish stromal melanin in the thin anterior border layer, which by its position has an overt influence on the overall color. The degree of dispersion of the melanin, which is in subcellular bundles called melanosomes, has some influence on the observed color, but melanosomes in the iris of humans and other vertebrates are not mobile, and the degree of pigment dispersion cannot be reversed. Abnormal clumping of melanosomes does occur in disease and may lead to irreversible changes in iris color (see heterochromia, below). Colors other than brown or black are due to selective reflection and absorption from the other stromal components. Sometimes lipofuscin, a yellow "wear and tear" pigment, also enters into the visible eye color, especially in aged or diseased green eyes.
   The sclera, also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some elastic fiber. In humans, the whole sclera is white, contrasting with the coloured iris, but in other mammals the visible part of the sclera matches the colour of the iris, so the white part does not normally show. In the development of the embryo, the sclera is derived from the neural crest. In children, it is thinner and shows some of the underlying pigment, appearing slightly blue. In the elderly, fatty deposits on the sclera can make it appear slightly yellow. Many people with dark skin have naturally darkened sclerae, the result of melanin pigmentation.
   Section::::Genetic determination.
   Section::::Mate selection and traits that have been linked to iris color.:Selection though imprinting of parental eye color.
  
  In contrast to the phenomenon of selection for rarity, scholarship has implied the existence another form of eye color involvement in mate selection. A study found a significant incidence of the partners of heterosexual people possessing similar eye and hair color to that of their opposite-sex parent. This is suggestive of a form of parental imprinting on eventual mate selection.
  
  Section::::See also.
  
  BULLET::::- Hair color
  
  BULLET::::- Limbal ring
  
  BULLET::::- Iridology
  
  BULLET::::- Human skin color
  
  BULLET::::- Xanthophore
  
  BULLET::::- List of Mendelian traits in humans
   BULLET::::- The vascular tunic (or uvea) is made up of the choroid, the ciliary body, and the iris. The choroid has a great deal of pigment, and is almost entirely made of blood vessels. It forms the tapetum lucidum when it crosses over the fundus of the eye, causing the yellowish-green eye shine when light is directed into the animal's eyes at night. The tapetum lucidum reflects light back onto the retina, allowing for greater absorption in dark conditions. The iris lies between the cornea and the lens, and not only gives the eye its color, ("see "eye color," below") but also allows varying amounts of light to pass through its center hole, the pupil.
   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.
   Section::::Pigmentation.
  
  The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related—although problems of convergence do exist.
  
  Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides.
  
  The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since.
   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.
   As early as 2004, evidence was compiled showing that the bulk of phenotypic variation in human iris color could be attributed to polymorphisms in the OCA2 gene. This paper, and the work it cited, laid the foundation for the inference of human iris color from DNA, first carried out on basic level by DNAPrint Genomics Beginning in 2009, academic groups developed and reported on more accurate predictive models for eye color and, more recently, hair color in the European population. More recently, companies such as Parabon NanoLabs and Identitas have begun offering forensic DNA phenotyping services for U.S. and international law enforcement.
   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.
   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.
   Section::::Eye color chart (Martin scale).:Brown.
  
  In humans, brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed.
  
  Dark brown eyes are dominant in humans and in many parts of the world, it is nearly the only iris color present. Dark pigment of brown eyes is common in Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa and the Americas. The majority of people in the world overall have brown eyes to dark brown eyes.
   The optical mechanisms by which the non-pigmented stromal components influence eye color are complex, and many erroneous statements exist in the literature. Simple selective absorption and reflection by biological molecules (hemoglobin in the blood vessels, collagen in the vessel and stroma) is the most important element. Rayleigh scattering and Tyndall scattering, (which also happen in the sky) and diffraction also occur. Raman scattering, and constructive interference, as in the feathers of birds, do not contribute to the color of the human eye, but interference phenomena are important in the brilliantly colored iris pigment cells (iridophores) in many animals. Interference effects can occur at both molecular and light microscopic scales, and are often associated (in melanin-bearing cells) with quasi-crystalline formations which enhance the optical effects. Interference is recognised by characteristic dependence of color on the angle of view, as seen in eyespots of some butterfly wings, although the chemical components remain the same.
   Those with lighter iris color have been found to have a higher prevalence of age-related macular degeneration (ARMD) than those with darker iris color; lighter eye color is also associated with an increased risk of ARMD progression. A gray iris may indicate the presence of a uveitis, and an increased risk of uveal melanoma has been found in those with blue, green or gray eyes. However, a study in 2000 suggests that people with dark brown eyes are at increased risk of developing cataracts and therefore should protect their eyes from direct exposure to sunlight.
  
  Section::::Medical implications.:Wilson's disease.
   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.
   Eye color outside of the iris may also be symptomatic of disease. Yellowing of the sclera (the "whites of the eyes") is associated with jaundice, and may be symptomatic of liver diseases such as cirrhosis or hepatitis. A blue coloration of the sclera may also be symptomatic of disease. In general, any sudden changes in the color of the sclera should be addressed by a medical professional.
  
  Section::::Medical implications.:Aniridia.
  
  Aniridia is a congenital condition characterized by an extremely underdeveloped iris, which appears absent on superficial examination.
  
  Section::::Medical implications.:Ocular albinism and eye color.
   A relative handful of genes accounts for the inherited factors shaping a person's appearance. Humans have an estimated 19,000-20,000 human protein-coding genes. Richard Sturm and David Duffy describe 11 genes that affect skin pigmentation and explain most variations in human skin color, the most significant of which are MC1R, ASIP, OCA2, and TYR. There is evidence that as many as 16 different genes could be responsible for eye color in humans; however, the main two genes associated with eye color variation are "OCA2" and "HERC2", and both are localized in Chromosome 15. 
  