Why is the human eye colour generally Brown, Blue and other similar variations. Why no bright green, purple, black or orange?
In: Biology
Actually ELY5: Color is what you see when light reflects off a surface. The melanin in your eye is the ‘surface’ light bounces from when you see eye color. Human eye melanin can’t reflect colors like bright green, black, or orange; only colors like brown, blue, grey, and the green we see.
Basically, it’s because our eyes don’t contain the pigments, nor the structures to enable us to have those eye colours.
The coloured parts of our eyes (the irises) are composed of two layers – the epithelium and the stroma.
The epithelium contains melanin; and the stroma is composed of colourless collagen fibres. Sometimes, the stroma can contain varying amounts of melanin too, and that’s really what dictates a person’s eye colour.
You know how if you look super closely at an eye, you can see darker lines/patches in the iris? That’s the epithelium showing through the stroma!
As melanin is the only pigment, you may have guessed that the colour we see is structural, and depends on the amount of light that reflects (refracts?) off the collagen fibres in our eyes… You’d be right.
In brown eyes, the stroma contains a high concentration of eaumelanin, which absorbs most of the light that enters into the eye, making it appear dark.
In hazel eyes, the stroma contains a moderate amount of eaumelanin. Because there’s less than in brown eyes, more light gets scattered back out of the eye, making them appear somewhat lighter.
In green eyes, the stroma contains a small amount of phenomelanin. Because there’s only a small amount of melanin in the stroma, more light gets scattered back out of the eye, meaning that they appear to be a lighter colour. The structure of the collagen fibres in the stroma also produce an effect similar to ‘The Tyndall Effect’ i.e. the ‘effect’ that makes the sky appear blue. The two things combined is what makes them appear green.
In blue eyes, there is no melanin in the stroma. Because of that, pretty much all of the light that enters into the eye is scattered back out, so the eyes appear bright. It’s also why blue eyes can appear to change colour in different light conditions.
Grey eyes are a little bit special… They are essentially the same as blue eyes (in that their stromas don’t contain melanin), but… They do contain extra deposits of collagen, which interferes with the whole Tyndall scattering thing, and means that all wavelengths of light entering into the eye are reflected equally.
Here’s a great link that has diagrams which makes all of this A LOT clearer, and A LOT easier to understand: https://medium.com/@ptvan/structural-eye-color-is-amazing-24f47723bf9a
Also, it’s alleged that Elizabeth Taylor actually had blue-grey eyes, and they appeared to be violet in certain lighting conditions (and were made to look even more violet, thanks to photograph editing).
Edit: I forgot to mention this interesting tidbit – you’ll NEVER see a blue, or a green cockatoo, because they simply lack the structure in their feathers to enable that. Yes, birds’ feathers rely on a good deal of structural colour too (although they can contain things like carotenoids and polyenal lipochromes/psittacofulvins, as well as melanin)… Cockatoos have rather archaic feathers that lack a spongey layer in their cortexes, and it’s that spongey layer that is responsible for them appearing blue, or green.
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