Eye colors can range from the most common color, brown, to the least common, green. Rare genetic specialties can even lead to unusual eye colors: black, red, and violet. Eye color is an inherited trait influenced by more than one gene. These genes are being sought using associations to small changes in the genes themselves and in neighboring genes. The actual number of genes that contribute to eye color is unknown at present, but there are a few likely candidates.

Classification of colors

The perception of color depends upon various factors. These are the same eyes; however, depending on the light and surrounding hues, the eye color can appear quite different.

Eye color exists on a continuum from the darkest shades of brown to the lightest shades of blue. Seeing the need for a standardized classification system that was simple, yet detailed enough for research purposes, Seddon et al developed a graded one based on the predominant iris color and the amount of brown or yellow pigment present. There are 3 true colors in the eyes that determine the outward appearance; brown, yellow, and blue. How much of each color one has determines the appearance of the eye color. The color of the eyes in turn depends on how much of these colors are present. For example, green eyes have yellow and some blue, making them appear green. Brown eyes appear brown because most of the eye contains the brown color. The above is true for Homo sapiens; the iris color can vary in the animal world. Instead of blue in humans, autosomal recessive color in the species Corucia zebrata is black, whereas the autosomal dominant color is yellow-green.

Amber

Human amber eyes displaying the yellow pigments.

Amber eyes are of a solid color and have a strong yellowish/golden and russet/coppery tint. This might be due to the deposition of the yellow pigment called "lipochrome" in the iris (which is also found in green and violet eyes). They are nicknamed "Wolf eyes" due to the high rate of the amber eye color in wolves. Amber eyes should not be confused with hazel eyes; although hazel eyes may contain specks of amber or gold, they usually tend to comprise of many other colors, including green, brown and orange. Also, hazel eyes may appear to shift in color and consist of flecks and ripples; while amber eyes are of a solid gold hue.

Blue

A blue eye

Blue eyes are the second most common eye color in the world, although they are very recessive.

Researchers conclude that the mutation may have arisen in a single individual around the Black Sea region 6,000-10,000 years ago, perhaps suggesting that all people with true blue eyes are more closely related. However, blue eyes with brown spots around the pupil are not related to this mutation.

Blue eyes are most common in Ireland, Poland , Netherlands, Iceland , Austria, Sweden, Norway, Denmark, Russia, Finland, Germany, France, Estonia and Latvia,, and the United Kingdom^. They are also present in Southern Europe, Spain, Portugal, Italy and the Balkans , the Middle East , and are also found in Afghanistan, Iran and north of India. A 2002 study found the prevalence of blue eye color among Whites in the United States to be 33.8% for those born from 1936 through 1951 compared with 57.4% for those born from 1899 through 1905.

Brown

Brown human iris

Light brown human iris

Brown eyes are predominant in humans and, in many populations, it is (with few exceptions) the only iris color present. It is less common in countries around the Baltic Sea, such as Finland, Estonia and Latvia.

In humans, brown eyes contain large amounts of melanin (melanin) within the iris stroma, which serves to absorb light, particularly at the shorter wavelengths. Brown eyes are the most common eye color, with over half of the world population having them. They are also the most dominant eye color gene.  Very dark brown irises may appear at a glance to be black.

 Gray

A steel blue-gray eye

Gray eyes have less melanin than blue eyes, even though they are considered a darker shade of blue (like blue-green). They are most common in European Russia, Finland and the Baltic States. Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Visually, gray eyes often tend to appear to change between the shades of blue, green and gray. This is said to be influenced by the lighting and the surroundings (such as clothes, makeup, etc.).

The Greek goddess Athena was renowned for having "owl-gray" or "sea-gray" eyes (in Greek, γλαυκῶπις – glaukōpis).

 Green

Green eyes

Green eyes are the product of light to moderate amounts of melanin. According to some researchers, green eyes are the result of mutations that change the melanin structure. Green eyes are the rarest natural eye color found in humans. Only 1-2% of the world population has green eyes. Green eyes are most common in northern Europe, Scandinavia, Germany, Ireland and sometimes can be found in France, Spain, Italy, to a lesser extent in the Middle East, as well as Northern parts of India, Pakistan and Afghanistan. It is also found within parts of South America, including Colombia. Much of the Icelandic population has either green or blue eye color.

Hazel

This eye shows a mixture of brown, green and amber colors.

Some eye colors are too mixed to identify properly, and are identified as hazel for simplicity's sake.

Hazel eyes are due to a combination of a Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from a light brown to a medium golden-green. A number of studies using three-point scales have assigned "hazel" to be the medium-color between the lightest shade of blue and darkest shade of brown. This can sometimes produce a multicolored iris, i.e., an eye that is light brown near the pupil and charcoal or amber/dark green on the outer part of the iris (and vice versa) when observed in sunlight. Hazel is commonly found in some regions of the Middle East, Europe, North America, parts of Central Asia, parts of North India, Northern Pakistan, Iran and in Afghanistan. Rarely, hazel eyes can be found in people of sub-saharan African, Native American and East Asian descent.

Definitions of the eye color "hazel" vary: it is sometimes considered to be synonymous with light-brown and gold. In North America, "hazel" is often used to describe eyes that appear to change color, ranging from light brown to green and even gray, depending on lighting in the environment.

Red

The eyes of a person with albinism may appear red under certain lighting conditions due to the very low quantities of melanin. "True" red eyes also exist in albinistic populations, but are very rare. 

The red-eye effect in flash photographs makes the pupils (rather than the irises) appear red. It is not related to eye color.

Violet

The appearance of "violet" eyes is thought to occur from the mixing of red and blue reflections. Some albinos have eyes that appear violet. Violet eyes are a form of blue eyes, and are just a reflection, pigment, or variant of blue. 

Medical implications

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. An increased risk of uveal melanoma has been found in those with blue, green or gray iris color.

Eye color may also be symptomatic of disease. Aside from the iris, yellowing of the whites of the eyes is associated with jaundice and symptomatic of liver disease, including cirrhosis, hepatitis and malaria.

Anomalous conditions

Aniridia

Aniridia: Eyes wherein the irises are not present; the eyes appear to be two large pupils.

Aniridia is a congenital condition characterized by an extremely underdeveloped iris which appears absent on superficial examination.

Ocular albinism and eye color

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 irises 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 total lack of melanin. 

Heterochromia

An example of heterochromia. The subject has one brown and one hazel eye.

An example of sectoral heterochromia. The subject has a blue iris with a brown section.

Heterochromia (also known as a heterochromia iridis or heterochromia iridium) is an ocular condition in which one iris is a different color from the other iris (complete heterochromia), or where the part of one iris is a different color from the remainder (partial heterochromia or sectoral heterochromia). It is a result of the relative excess or lack of pigment within an iris or part of an iris, which may be inherited or acquired by disease or injury. This uncommon condition usually results due to uneven melanin content. A number of causes are responsible, including genetics such as chimerism and Waardenburg syndrome. Trauma and certain medications, such as some prostaglandin analogues can also cause increased or decreased pigmentation in one eye. On occasion, the condition of having two different colored eyes is caused by blood staining the iris after sustaining injury.

British singer David Bowie is a famous person often wrongly attributed with heterochromia. His apparent condition is due to a teenage injury. (One eye appears darker because the pupil is permanently dilated.) American actress Kate Bosworth has sectoral heterochromia, resulting in a hazel section at the bottom of her right blue eye, while the left is completely blue. American Actress Elizabeth Berkley has sectoral heterochromia; her right eye is half green and half brown, and her entire left eye is green. So does actor Anthony Head - he has a patch of hazel in his left eye where both eyes are blue-green overall. The lead vocalist of American band Rise Against, Tim McIlrath, has heterochromia; his left eye is blue while his right is brown. American actress Mila Kunis also has heterochromia, resulting in one blue eye and one brown-green eye. American Actress Demi Moore also has heterochromia, by having one green eye, and the other hazel. The English Actress Jane Seymour also has heterochromia; her left eye is green whereas her right eye is brown.

Eye color change

Often, Caucasian newborns have blue eyes, which change to green, hazel, light brown or dark brown. This is possibly the origin of the idiom "being blue-eyed" (i. e. naïve; gullible).

It is thought that exposure to light after birth triggers the production of melanin in the iris of the eye. By three years of age, the eyes produce and store enough melanin to indicate their natural shade. While changes in eye color of infants are more common, even in adults, eye color changes are seen, most often as a result of exposure to the sun. Sunlight triggers melanin production in the eye, as it does to the skin.