Why can’t human eyes be all colors of the rainbow?

Why can’t human eyes be all colors of the rainbow? Why are they restricted to brown/ blue/ green (and sometimes red) as opposed to other colors?

I found a pretty good article which answered my question. I understood this:

Eye color is in the iris and the iris has two layers:

  • The epithelium (the back layer) – contains black-brown pigment.
  • The stroma (the front layer) – made up of colorless collagen fibers. The stroma can sometimes contain a pigment called melanin and it can also sometimes contain excess collagen deposits. These are the two factors that control eye color.

Eye colors

Brown eyes contain a large amount of melanin in their stroma. Melanin absorbs most of the light entering the eye and therefore gives the eyes a dark color.
(Melanin in itself is dark brown. It can also be found in hair/skin – people with darker skin/hair have more melanin than people with lighter skin/hair).

Blue eyes have a completely colourless stroma. This means that blue eyes aren’t blue because they contain blue pigmented cells! All the light that enter the eyes is actually scattered back into the atmosphere and the eyes only look blue to people because of a phenomena called the Tyndall effect. This effect is also the reason why we see the sky as blue (here are two videos that explain this well: video 1, video 2 – it’s actually super interesting!). This also explains why the blue color in the eyes depends on the quantity/quality of the light available in the room.

Green eyes contain a small amount of melanin in their stroma.
: mix of Tyndall effect which creates the blue + melanin which creates the brown = green

– Just like blue eyes, grey eyes also have a completely colorless stroma. However, they also have excess collagen deposits in the stroma which interferes with Tyndall scattering and blocks the appearance of the “blue”. This is why the eyes look grey.
(This is, however, just a theory).

– Finally, people with Albinism have red eyes. This is because their body can’t produce the melanin pigment at all. This means that both the stroma and the epithelium are colorless. The eyes appear red because red is the color of the retina in the back of the eye. Interestingly, it is also why we sometimes have red eyes in photos – flash photography causes you to accidentally image peoples retinas. This also explains why people with Albinism usually have poor vision. The purpose of the iris is to block out light like a camera aperture. However, without pigment, the iris can’t block out the light like it is supposed to.

Other animals such as birds can have a wider range of eye colors because they actually have a wider variety of pigments in their eyes.

(two images from the article)

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