Brown eyes
n 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.

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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, South Europe, East Asia, Southeast Asia, Central Asia, South Asia, West Asia, Oceania, Africa, Americas, etc. as well as parts of Eastern Europe and Southern Europe. The majority of people in the world overall have brown eyes to dark brown eyes.Brown eyes are actually blue underneath.

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Light or medium-pigmented brown eyes can also be commonly found in South Europe, among the Americas, and parts of Central Asia (Middle East and South Asia). Due to Han Chinese, and Indian being the largest Race/Ethnic group in the world it make sense that dark eyes is common in worldwide population to be honest.

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Gray

Like blue eyes, gray eyes have a dark epithelium at the back of the iris and a relatively clear stroma at the front. One possible explanation for the difference in the appearance of gray and blue eyes is that gray eyes have larger deposits of collagen in the stroma, so that the light that is reflected from the epithelium undergoes Mie scattering (which is not strongly frequency-dependent) rather than Rayleigh scattering (in which shorter wavelengths of light are scattered more). This would be analogous to the change in the color of the sky, from the blue given by the Rayleigh scattering of sunlight by small gas molecules when the sky is clear, to the gray caused by Mie scattering of large water droplets when the sky is cloudy. Alternatively, it has been suggested that gray and blue eyes might differ in the concentration of melanin at the front of the stroma.

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Gray eyes are most common in Northern and Eastern Europe. Gray eyes can also be found among the Algerian Shawia people of the Aurès Mountains in Northwest Africa, in the Middle East, Central Asia, and South Asia. Under magnification, gray eyes exhibit small amounts of yellow and brown color in the iris.

Green

As with blue eyes, the color of green eyes does not result simply from the pigmentation of the iris. The green color is caused by the combination of: 1) an amber or light brown pigmentation in the stroma of the iris (which has a low or moderate concentration of melanin) with: 2) a blue shade created by the Rayleigh scattering of reflected light.Green eyes contain the yellowish pigment lipochrome.

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Green eyes probably result from the interaction of multiple variants within the OCA2 and other genes. They were present in south Siberia during the Bronze Age.

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They are most common in Northern, Western and Central Europe. In Ireland and Scotland 14% of people have brown eyes and 86% have either blue or green eyes, In Iceland, 89% of women and 87% of men have either blue or green eye color. A study of Icelandic and Dutch adults found green eyes to be much more prevalent in women than in men.[68] Among European Americans, green eyes are most common among those of recent Celtic and Germanic ancestry, about 16%. 37.2% of Italians from Verona and 56% of Slovenes have blue/green eyes.

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Green eyes are common in Tabby cats as well as the Chinchilla Longhair and its shorthaired equivalents are notable for their black-rimmed sea-green eyes.

Hazel
Hazel eyes are due to a combination of Rayleigh scattering and a moderate amount of melanin in the iris' anterior border layer. Hazel eyes often appear to shift in color from a brown to a green. Although hazel mostly consists of brown and green, the dominant color in the eye can either be brown/gold or green. This is how many people mistake hazel eyes to be amber and vice versa. This can sometimes produce a multicolored iris, i.e., an eye that is light brown/amber near the pupil and charcoal or dark green on the outer part of the iris (or vice versa) when observed in sunlight.

Definitions of the eye color hazel vary: it is sometimes considered to be synonymous with light brown or gold, as in the color of a hazelnut shell.

Hazel eyes are common throughout Caucasoid populations, in particular in regions where blue, green and brown eyed peoples are intermixed.

Blue

There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin. Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.The result is a "Tyndall blue" structural color that varies with external lighting conditions.

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There is no blue pigmentation either in the iris or in the ocular fluid. Dissection reveals that the iris pigment epithelium is brownish black due to the presence of melanin. Unlike brown eyes, blue eyes have low concentrations of melanin in the stroma of the iris, which lies in front of the dark epithelium. Longer wavelengths of light tend to be absorbed by the dark underlying epithelium, while shorter wavelengths are reflected and undergo Rayleigh scattering in the turbid medium of the stroma. This is the same frequency-dependence of scattering that accounts for the blue appearance of the sky.The result is a "Tyndall blue" structural color that varies with external lighting conditions.

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Girl here is Norwegian(Norway have an high percentage of Blue eyes)

In humans, the inheritance pattern followed by blue eyes is considered similar to that of a recessive trait (in general, eye color inheritance is considered a polygenic trait, meaning that it is controlled by the interactions of several genes, not just one)
A 2002 study found that the prevalence of blue eye color among the white population in the United States to be 33.8% for those born from 1936 through 1951 compared with 57.4 percent for those born from 1899 through 1905.

Here’s an interesting statistic about the distribution of eye color. Please note that this information relates to a specific population and I do not have the actual makeup of that group.

Percentage
32% blue/grey irises
15% blue/grey/green irises with brown/yellow specks
12% green/light brown irises with minimal specks
16% brown irises with specks
25% dark brown irises

Soo yeah here is the percentage of Blue eyes

89%
Estonia

89%
Ireland

57%
Scotland

50%
England

48%
Wales

45%
Belgium (adult population)

28.9%
France (adult pop.)

20.2%
United States

16.6%
Spain

16.3%
Algeria (adult pop.)

2.6%
Morocco (adult pop.)

2.1%
Tunisia (adult pop.)

1.2%
sources(Like West Africa, Arab countries etc)

Hazel eyes have a higher concentration of melanin (pigment) around the eye's border, which can result in a multi-colored appearance that varies between copper and green depending on the lighting. Most people estimate that around 5-8% of the world's population has hazel colored eyes.

Rarity

No two pair of hazel eyes is exactly the same. Some people have lighter features of green while others have darker features of brown. Ocular scientists think hazel eye color is a combination of greens, browns and blues but if truth be told, they aren't exactly sure. We do know however, that hazel is considered rare.