The colored part of your eye is called the iris. It’s the ring-shaped structure surrounding the black center (the pupil), and its color comes from a pigment called melanin distributed across multiple layers of tissue. The specific shade you see depends on how much melanin is present and how light interacts with the iris’s internal structure.
How the Iris Creates Color
The iris is made up of several layers, but two matter most for color. The back layer, called the epithelium, is only two cells thick and contains dark brown-black pigment in almost everyone, regardless of eye color. The front layer, called the stroma, is where the differences show up. The stroma contains collagen fibers, blood vessels, and pigment-producing cells called melanocytes. How many melanocytes are present and how much melanin they produce determines whether your eyes appear brown, green, blue, or somewhere in between.
Brown eyes have large amounts of melanin in the stroma. The pigment absorbs most incoming light, giving the iris its rich, dark appearance. More than 50% of the world’s population has brown eyes, making it by far the most common color.
Blue eyes work completely differently. People with blue eyes have essentially no pigment in the stroma at all. Instead, the colorless collagen fibers scatter incoming light back out through a process called the Tyndall effect, similar to the Rayleigh scattering that makes the sky appear blue. Blue eye color is entirely structural, meaning there is no blue pigment anywhere in the eye.
Green eyes sit in between. The stroma has a small amount of melanin but no collagen deposits. Light scattering from the Tyndall effect produces a blue hue, and when that blue mixes with the small amount of brown melanin, the result looks green. Only about 2% of people worldwide have green eyes, making it the rarest common eye color.
Hazel eyes follow a similar principle but with more melanin than green eyes, creating a shifting mix of brown, gold, and green that can appear to change depending on lighting conditions.
Why Eye Color Can Look Different Day to Day
The iris also contains muscles. A circular sphincter muscle near the pupil constricts to limit light, and radial dilator muscles expand the pupil in dim conditions. When your pupil changes size, it compresses or spreads the pigment in the stroma, which can make your eye color appear slightly lighter or darker. Lighting conditions, the colors you’re wearing, and even crying (which reddens the surrounding tissue) can shift how your iris color is perceived, even though the actual pigment hasn’t changed.
Eye Color Changes Over a Lifetime
Many babies are born with blue or gray eyes because melanin production in the stroma ramps up gradually. Most children settle into their permanent eye color by age three, though it can take longer. In about 10 to 15% of white adults, eye color continues to shift subtly past early childhood and even into adulthood, typically darkening slightly as melanin accumulates.
Certain glaucoma eye drops can also change iris color permanently. These medications, which belong to a class called prostaglandins, stimulate melanin production in the iris. When only one eye is being treated, this can create a noticeable color difference between the two eyes that may be permanent or only very slowly reversible.
Heterochromia: When Eyes Don’t Match
Having two different-colored eyes, or color variations within a single eye, is called heterochromia. It comes in three forms:
- Complete heterochromia: Each eye is a completely different color, such as one brown and one blue.
- Sectoral heterochromia: One iris has a wedge-shaped section of a different color, like a pie slice that doesn’t match the rest.
- Central heterochromia: The inner ring around the pupil is a different color from the outer iris, often appearing as spikes radiating outward from the center.
Most cases are caused by harmless genetic mutations that affect how melanin is distributed during development. Central heterochromia in particular is fairly common and often goes unnoticed unless you look closely in a mirror. Less commonly, heterochromia can result from eye injuries, inflammation, or certain medical conditions. When color changes appear suddenly in one eye during adulthood, it’s worth having it evaluated, since acquired heterochromia can occasionally signal an underlying problem.
What Determines Your Eye Color Genetically
Eye color was once taught as a simple dominant-recessive trait, with brown always overriding blue. The reality is more complex. At least 16 genes influence iris color, which is why two blue-eyed parents can occasionally have a brown-eyed child, and why eye colors like hazel and green don’t fit neatly into a two-gene model. The primary gene controlling melanin production in the iris sits on chromosome 15, but variations across many other genes fine-tune the final result. This is also why siblings with the same parents can end up with noticeably different eye colors.