Human eyes come in many colors — brown, blue, green, hazel, amber, and even violet or gray eyes. Gray eye color is one of the loveliest and most uncommon, a trait shared by only 3% of the world’s population. The color and intensity of gray eyes varies from person to person and can include dark gray, gray-green and gray-blue.

Eye color actually refers to the color of the iris, a ring of tissue that surrounds the pupil. The pupil is an opening at the center of the iris that appears black, while the white part of your eye is called the sclera.

The color of the iris depends on the presence of a brown pigment called melanin, the same pigment that determines skin color and hair color. Eyes with a lot of melanin are darker, and eyes with less melanin are blue, green, hazel, amber or gray.

NOTE: You may see it referred to as a “grey eye” rather than a “gray eye,” but it’s the same eye color. “Gray” is simply the preferred spelling in American English, while “grey” is the British English spelling used primarily in the U.K.

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Gray eyes are neither recessive nor dominant. Scientists used to think that a person’s eye color was caused by one dominant gene, and that darker eyes (like brown eyes) were dominant; while lighter eyes (blue eyes, green eyes, hazel eyes and gray eyes) were recessive.

A recessive gene only shows up when there are two copies of it present. It was thought that if you inherited one gene for brown eyes and one gene for blue eyes, the gene for brown eyes would dominate and both of your eyes would be brown.

Today scientists know this is not the case and that many genes play a role in how eye color develops. Most of those genes help regulate melanin.

Gray eyes can appear in various shades, including dark gray, gray-blue, gray-green or almost hazel. The intensity of gray eyes depends on the individual.

People with gray eyes may also note that their eyes seem to change color depending on the color of clothing (or eye makeup) the individual is wearing, or how bright the surrounding lighting is.

Even mood can appear to change the tint of gray, because pupils dilate (open wider) when someone is experiencing extreme emotions, such as grief or joy. When that happens, gray eyes can appear darker — though of course the eyes aren’t actually a different color. This effect can also happen with certain medications, such as opioids, which enlarge the pupils.

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In all eyes, the amount of melanin in the iris regulates eye color. This brown pigment absorbs light, and the color of the eye depends in large part on how much melanin the eye contains. Every iris contains two layers of tissue, one in front and one in back, joined by connective tissue in the middle called the stroma.

Dark eyes contain a lot of melanin in both the front and back layers of the iris. Very little light is reflected back out, which is why the eyes appear brown or black. In lighter eyes, there is less melanin, and it is located in the back layer.

Though scientists don’t yet know exactly what causes gray eye color, they believe that the genetics at work are likely the same as — or very similar to — the genetics behind the development of blue eyes.

Gray eyes may contain just enough melanin in the front layer to dim the blue wavelengths of light that are reflected back by the tissue of the eye. Dark gray eyes have a bit more melanin in that front layer than pale gray eyes.

Scientists also think that the fibers in the stroma may scatter light in such a way that the iris appears gray.

When melanin pigment absorbs light, it helps protect the eyes. People with light colored eyes (blue, green or gray) are more likely to be sensitive to bright light. If this describes you, you can protect your eyes with UV-blocking polarized sunglasses and wide-brimmed hats on bright sunny days.

The more melanin your iris contains, the more protected you are from the sun’s damaging rays. Individuals with gray eyes have less melanin and are at greater risk for an eye cancer called ocular melanoma.

This cancer is very rare, affecting six in every one million adults in the U.S. annually, but it’s a good idea to wear those UV-blocking sunglasses in any case.

If you’ve got gray eyes, you are less likely to suffer from a skin disorder called vitiligo, a condition in which your immune system attacks cells with melanin pigment and leaves behind irregular patches of white skin.

Vitiligo is an autoimmune disease, and it’s associated with a higher incidence of Type 1 diabetes, autoimmune thyroid disease, rheumatoid arthritis and lupus. Researchers think that people with gray eyes are less likely to get all these autoimmune diseases.

People with light-colored eyes, including gray eyes, consume more alcohol than those with dark eyes, according to a 2001 study that looked at data from two surveys of over 12,000 individuals. Meanwhile, those with dark eyes are more sensitive to alcohol and become intoxicated more easily — drinking less as a result.

Similarly, people with gray eyes may be less sensitive to medications than those with darker eyes. Studies dating from the 1970s and 1980s confirm an overall greater sensitivity to stimuli in participants with dark eyes.

Even giving birth seems to cause less pain and anxiety to those with light-colored eyes, such as blue or gray eyes.

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