You’ve seen black, brown, blue or green colored eyes of people. But you never see someone with red or purple eyes. What could be the reason for this?
The following two answers explain why human eye colors are limited to only a few colors.
Eye color is in the Iris, and the Iris has two layers, a back layer that’s always fully pigmented, and a thicker front layer. The pigment is melanin, same that makes our skin different colors. Now when the front layer is densely pigmented, it appears dark, like brown or even black (though shine a bright enough light and you’ll see black eyes are just very dark brown).
If it’s not pigmented, it appears light blue. Why? Same reason the sky’s blue, light scatters in it. Light scattering is a different topic, but basically short wavelengths (ie blue) bounce differently in the fluffy front layer without pigment.What about in between? Well it turns out if there’s pigment but it’s not super dense, it’s a bit of a lighter brown or dark orange. We call these amber eyes.
If it’s between amber and blue, then it’s like a cross between light orange and blue. That’s how you get green eyes.So that’s how you get the spectrum: from blue to green to amber to brown to black.
So what about “red” or “violet” eyes? Turns out I lied in the beginning; that back layer I mentioned that’s always pigmented isn’t in a medical condition: Albinism. These folks unfortunately have a defect in producing melanin pigment, which is why they all have fair skin and light hair and such.
So why red? It’s the same reason you can get red eyes in flash photography; the red is the color of the retina in the back of the eye. Flash photography causes you to accidentally image peoples retinas.
Now without the pigment in the back layer of the iris, the iris almost functionally becomes like Saran Wrap; it’s clear and doesn’t block out light anymore. So when you see red (or violet) eyes, you’re seeing retina through their Iris.
Side note: this is one reason why folks with albinism have poor vision. The purpose of the iris is to act like a camera aperature. Without pigment, it can’t block out light like an aperature, removing a whole element of the focusing system of the eye.
Disclaimer: this is a bit of an oversimplification of how eye colors work, but it’s fundamentally not too far off. The genetics that go into eye color get very complex, don’t think it’s some single gene thing; I know this explanation makes it seem like there’s just one toggle, but that’s not how the genetic part of it works.
There are some rarer variations of colors outside of brown, blue and green.
First we need to look at why human eyes have any color. There’s a layer in your eye that contains pigment. The most common pigment is melanin. Melanin itself is dark brown. Melanin is also found in hair and skin – people with more melanin in their skin have darker skin, and those with less melanin are more fair.
Those who have very little melanin have blue eyes. Why is this? There isn’t a “blue” pigment that’s in blue eyes. I’m not well equipped to explain this but it’s the same reason why the sky appears blue. It has to do with blue light waves (there’s all sorts of different color wavelengths) bouncing off oxygen and nitrogen particles in the air. The blue ones are realllly good at this, which is why we see blue. The Tyndall effect is what best fits why eyes with low/no melanin are blue. Light enters, long wavelengths (aka non-blue wavelengths) are long enough to reach the back of the eye. The back of our eyes have the ability to absorb light. So basically all the long wavelengths get absorbed! However, blue is a SHORT wavelength and doesn’t reach the back very well. It bounces around and ultimately gets reflected back out. That’s why we see some people’s eyes as blue. Remember, we see what is reflected, NOT absorbed. For example, a tomato appears red because it absorbed all other light wavelengths except red. The red wavelength is reflected – it’s what we see.
It’s actually pretty interesting, there’s a laser that’s still being approved that destroys the pigment in the iris to reveal blue eyes. Technically we all would have blue eyes if it weren’t for pigment. Originally, everyone had brown eyes, blue eyes were a gene mutation that popped up after a while.
But what about grey eyes? They’re…kinda blue, right? Well, there’s an idea that this difference is due to some people having more collagen (just a type of building block for stuff like skin and tissue!) in their eyes than other people. This can affect the whole light reflection stuff and give variation. This, along with eye shape (the angle you reflect the light is involved in the ‘clearness’ of the color) could also explain why some people have eye colors that really kind of “glow” and “pop” while others with similar eye colors don’t have very attention-drawing eyes. Ultimately, we still don’t have a truly clear answer as to why some people have bright blue eyes while others might have blue eyes on the duller, grey side. They both have low levels of melanin in their eyes but there’s a lot of other factors at play.
Now getting back to the other colors. Really dark brown eyes have a lot of melanin. Sometimes they even look black, but if you took a light to them you would see that it’s really just a deep brown color. More pigment = more absorbed wavelengths. So unlike blue eyes, light wavelengths coming in aren’t really getting out. You’re actually seeing melanin (that brown colored pigment mentioned earlier). Those with really high levels of it will have dark brown eyes, while those with more moderate amounts of it might have light brown eyes.
Okay…but what about GREEN? There’s a green pigment, right? Well, no. We need to look at what the differences and similarities to hazel and green eyes are to further understand. Hazel eyes involve having both the blue eye effects yet enough pigment to where the color seen isn’t blue. So there’s little enough melanin to still get some of those blue wavelengths to bounce around and get reflected out again, but there’s enough melanin to where you’re still seeing brown. Together, you get kind of a brownish green. There’s a lot of variations of this of course – hazel eyes are going to look different from person to person due to amounts of melanin and those other possible factors we mentioned in the gray vs blue eyes.
True green is actually quite rare. This is why: You have the same stuff as hazel eyes going on (low-ish melanin, blue light reflection) but you also have a decent amount of a different type of pigment called lipochrome. Lipochrome isn’t found as often in humans as other mammals like dogs, cats, etc. Melanin is BROWN, but lipochrome is YELLOW. Low-ish amounts of melanin, plus a good amount of lipochrome is what gives people green eyes. The “greenest” looking eyes would likely be ones with very very small amounts of melanin, but moderate amounts of lipochrome. The more melanin mixed in, the more brown-green (closer to hazel) you get instead of blue-green.
Those with a lot of lipochrome and melanin might have closer to amber-colored eyes.Even blue eyed people can have “flecks of gold” – this would likely be due to some bits of lipochrome in their eyes.
Other animals like birds can have a wider, more vivid range of colors than humans because they actually have a wider variety of pigments in their eyes. You’ll never see a human with the eye color of a great horned owl because we don’t produce a yellow pigment in our eyes that vibrant and bold. Lipochrome’s yellowness just doesn’t compare to some of those bright yellow pigments birds have!