Having eyes that appear to change color is a rare and fascinating trait. The apparent color change is caused by a number of factors relating to the melanin, collagen, and anatomy of the eye. While some eye colors may appear to shift between shades, only a few people have truly chameleon-like eyes that drastically shift between multiple colors. The reasons for and implications of color changing eyes are intriguing for those who have them as well as eye researchers and observers.
What Causes Eyes to Change Color?
There are a few different causes of color changing eyes:
Melanin Concentration
The pigment melanin plays a major role in eye color. Eyes with less melanin appear blue or light gray, while increased melanin makes eyes darker. Some people have variable amounts of melanin in their eyes that can change over time or adjust based on lighting conditions and pupil dilation. This can lead to eyes appearing lighter in bright settings and darker in dim lighting.
Rayleigh Scattering
Shorter wavelengths of light are scattered more easily than longer wavelengths. This selective Rayleigh scattering affects the color of the sky as well as eyes. The type and amount of melanin in the iris determines which colors are absorbed. The remaining scattered wavelengths are what give eyes their hue. This process can cause eyes to shift toward blue, green, or gray.
Collagen Density
The collagen content and density of the cornea and stroma layer covering the iris affects the way light scatters and reflects in the eye. Denser collagen leads to increased light scattering and lighter eye colors. As collagen density changes with age, some eyes seem to lighten over time.
Layering of Melanin and Collagen
Unique patterns in the layering and distribution of melanin and collagen contribute to striking color variations. In sectoral heterochromia, melanin is not evenly distributed, causing eyes to display multiple colors at once. Eyes can also appear to shift between shades of brown, green, or blue based on the overlapping layers of pigment and collagen.
Lighting Conditions
The colors we perceive depend on both the lighting and the object being observed. Since the hue and intensity of lighting changes, the apparent color of eyes can seem to shift throughout the day. Bright outdoor light versus dim indoor illumination can make the same eyes look lighter or darker.
Categories of Color Changing Eyes
There are a few main categories of color changing eyes:
Complete Heterochromia
Each iris is a distinctly different color. Most often this is caused by uneven melanin but can also relate to conditions like Fuch’s heterochromic iridocyclitis. Complete heterochromia is rare, estimated to occur in less than 1% of the population.
Sectoral Heterochromia
Only parts or sectors of the iris appear different colors. This is caused by uneven distribution of melanin. Sectoral heterochromia is also uncommon and present in an estimated 1% of people.
Central Heterochromia
The inner ring around the pupil appears a different color than the outer part of the iris. This effect is created by concentrated melanin around the pupil. Central heterochromia is more common than complete or sectoral types. About 5% of the population has central heterochromia.
Minor Variations
Most eyes with apparent color changes fall into this category. They may display lighter and darker shades of the same color or subtly shift between similar hues. This can be caused by melanin levels responding to pupil dilation, varied collagen density, or changes in lighting. Such minor variation affects a significant portion of the population.
Extent of Color Change
The extent to which eyes appear to shift color depends on the anatomical factors causing the change:
| Cause of Color Change | Extent of Apparent Color Shift |
| Uneven melanin distribution | Multicolored irises in heterochromia |
| Changing melanin concentration | Medium – Different shades of the same hue |
| Varying collagen density | Medium – Subtle lightening over time |
| Pupil response to light | Low – Slight darkening in bright light |
| Different lighting | Low – More blue or gray in dim conditions |
As seen above, the most dramatic shifts are attributed to uneven melanin causing heterochromia. Changes in melanin levels, collagen density, pupil dilation, and lighting cause more subtle effects.
Rarity of Color Changing Eyes
Truly notable color changing eyes are unusual. Central heterochromia is the most common form, occurring in an estimated 300 million people globally. This condition only creates a minor two-tone effect. Complete heterochromia with totally different colored irises is one of the rarest eye traits, likely affecting less than 1% of the population. The majority of eyes appear to change color based on small variations in melanin, collagen, pupil size, and lighting. So while many people’s eyes can look slightly different shades in certain conditions, very few have remarkable chameleon-like shifts in hue and intensity.
Implications of Color Changing Eyes
Variable eye color does not have any negative health effects. Some key implications of color changing eyes include:
Genetics
Certain hereditary factors cause the melanin and collagen irregularities behind color changing eyes. These are typically recessive traits that can be passed down to children.
Vision Ability
There are no impacts on visual acuity or ability. Color changes in the iris do not affect eyesight.
Sensitivity to Light
Increased melanin can make eyes more sensitive to bright light. This may lead some people with darker eye pigmentation to prefer wearing sunglasses outside.
Social Interest
Unusual heterochromia eye colors tend to elicit interest and curiosity. Having remarkable color changing eyes can draw attention and questions.
Identification and Security
Multicolored irises with distinctive patterns can be beneficial for biometric identification and security purposes. The unique traits of color changing eyes can allow for specialized access control based on iris scanning technology.
Notable People with Color Changing Eyes
A few celebrities and public figures have notable heterochromia or color changing eyes:
- Elizabeth Berkley – American actress with sectoral heterochromia
- Dan Aykroyd – Comedian with complete heterochromia
- Kate Bosworth – Actress with sectoral heterochromia
- Alice Eve – British actress with sectoral heterochromia
- Mila Kunis – Hollywood star with minor heterochromia
- David Bowie – Iconic musician with permanent dilated pupil and color variance
These and other stars showcase some of the most striking examples of color changing irises. Their eye traits have added to their intrigue and distinctive looks.
Conclusion
In review, eye color variation occurs due to factors like melanin concentration, collagen density, pupil dilation, and lighting conditions. Complete heterochromia is very rare, but sectoral and central forms affect a small portion of the population. Most color changing eyes exhibit subtle shifts rather than dramatic differences. The extent of change relates to the underlying anatomical and optical causes. Color shifting irises have little effect on vision but can draw attention and interest. In summary, variable eye color is an intriguing trait that reveals the complexities of anatomy, physics, and genetics at work in the human eye.