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I am writing a medieval fantasy story in which there is a race of humanoids who have multicolored eyes. Their eyes may change to bring out one color or another if they are experiencing very stressful/emotional situations. These humanoids are nearly the same as regular humans, except for their eyes, and they have a strong connection towards nature.

I was wondering if this eye change would be possible and whether or not it would have any impact upon a person's ability to see.

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    $\begingroup$ While it's more subtle, plenty of humans have eyes that change color. My daughter and I both do, for example. I can go from clear blue to strong green to hazel, and everything in-between. Zero change in vision. Makes no difference at all. $\endgroup$ – Cyn Apr 20 at 19:21
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    $\begingroup$ The color of the iris affects vision in the same way that the color of the camera affects photographs. The iris is an (automatically) adjustable diaphragm, it is opaque and limits the amount of light entering the eye. $\endgroup$ – AlexP Apr 20 at 20:03
  • $\begingroup$ Eye color anyways is the reflection that other people see, so unless the reflection is that strong I wouldn’t think. That said, you can use eye color to differentiate levels of effects these humanoid eyes have (water vision, sky vision, etc) for your target audience $\endgroup$ – clifton_h Apr 25 at 17:10
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Heterochromia

Some humans in our world have multi-coloured eyes, known as heterochromia. There are various different types, full, sectional and central. This happens due to various amounts of melanin being produced in the eyes. People may be born with heterochromia at birth or it may develop some time afterwards. It is possible to have multiple types of heterochromia.

Full or complete heterochromia is where a person has two different coloured eyes, such as one blue and one green. This is the most visibly obvious of the three and the one people commonly associate heterochromia.

Sectional or partial heterochromia is where a small segment of the eye is a different colour than the rest of it. Often it only affects one eye.

Finally central heterochromia is where there is a ring of colour around the iris which is different than the rest of the eye. To use myself as an example, my eyes are primarily green but there is a ring of brown that looks like it bursts out from the centre. Often it affects both eyes but it is possible that it could only affect one.

As someone with a form of heterochromia, I can safely say that having two different colours in my eyes does not affect my vision in the slightest. However, I have noticed my eyes are more photosensitive than those of my peers. I don’t know whether that's because of the heterochromia, simply because they are in dark colour or because I spend most of my time indoors in a dimly lit room behind a screen, though I would assume it is the latter.

Applying to your Question

Your question is essentially asking for an active version of heterochromia, in our world it doesn't change, your eyes remain the same colours, you don’t wake up one day with brown eyes and the next with green (however, as the pupils dilate and contract, more or less of the colour is exposed. In my case, it's hard to see the brown if my pupils are fully dilated and easier if they are fully contracted).

It is possible that your humanoids can actively (if subconsciously) add or remove melanin from the eyes. This would cause them to change colours. Certain emotions may dictate where the melanin goes, such as full heterochromia to show confusion or changing the eyes to a dark colour to show anger or fear. Using heterochromia as your basis, you could get some pretty complex emotions based on the colours and/or patterns.

Below are some examples of heterochromia:

(Note that although most people in these images are white, heterochromia can be found in every ethnic group. Heterochromia can also be found in some domesticated animals, such as cats and dogs.)

Eye with heterochromia https://steemit.com/life/@dkmilon/there-are-actually-three-types-of-heterochromia-for-eyes-i-found-rocks-that-match-it

Six eyes with heterochromia http://www.daltonism.org.uk/2018/04/human-eye-color/

Six eyes of different colors https://www.zmescience.com/science/why-eyes-colored-04322/

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No difference

Vision happens through the pupil, the dark "spot" in the centre of the eye. This is actually a clear window which light enters and through which it passes, through the lens & vitreous humour on its way to the retina at the back inner surface of the eye.

The coloured part is the iris and doesn't affect vision per se. The iris is muscular and contracts to lessen the amount of light passing through the pupil and relaxes to allow more light to pass through.

Iris colour is not relevant to vision.

As for the question of colour change being possible, of course! It's your world, you make up the rules!

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    $\begingroup$ I could believe that darker colors make it easier to see in the same way athletes darkening their cheeks does - but I could also believe the total effect, if it exists, isn't significant (otherwise evolution would have favored dark brown eyes). Theoretically you could claim the reverse, light-colored eyes would be more reflective of light and a benefit to night vision. But, again, evolution doesn't appear to demonstrate this benefit. But, as you say, "make up the rules!" $\endgroup$ – JBH Apr 20 at 19:35
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    $\begingroup$ @JBH -- Evolution is odd. Many deep sea fish have brown eyes. Why, if there is no sunlight to redirect? I think if archaic primates, long long ago, had been positively affected by pigmented zygomatic patches we'd have those by now. (An interesting feature for a fantasy race, that!) At best eye black (such as used by athletes) helps reduce glare a little bit. Doesn't help visual acuity. $\endgroup$ – elemtilas Apr 20 at 21:45
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    $\begingroup$ Yes. Anecdotal only... in my family about half of us have brown eyes and the other have have blue. I know for a fact that the blue eyed folks have ridiculously good night vision. We poor brown eyed people are always bumping into things in the middle of the night even though it doesn't seem to bother the people with blue eyes. On the other hand, I don't seem to need sunglasses unless it's really bright out, and my family members with blue eyes are always rocking those shades. So there you go. A real world example where eye color seems to make a difference. $\endgroup$ – MParm Apr 21 at 22:58
  • $\begingroup$ @MParm Do we have some stats for that? I'm blue/green and have terrible night vision. My brown-eyed spouse's night vision is just fine (we're the same age). We also have the same need for sunglasses. I'd like to see some data before accepting your conclusions. $\endgroup$ – Cyn Apr 22 at 1:46
  • $\begingroup$ @Cyn -- They did say "anecdotally". . . Low light vision is a factor of receptors in your retina. This is a matter for genetic make up rather than eye colour. $\endgroup$ – elemtilas Apr 22 at 13:10
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Squid change color quite spectacularly by contracting and widening chromatophores, basically small spots of color that sit side by side. As you can see from classic prints, by positioning red, blue and green dots side by side, and varying their size, you can create the impression of most colors of the spectrum.

There might even be 'special effects' by having some chromatophores be filled with a fluorescing or even phosphoresing color, or something reflecting specular, creating the impression of metal.

Sight need not be impaired, or even affected by the color change. The 'color' of humans' eyes is defined by the iris, around the pupil. The pupil is the part that lets light pass, and if the light is then absorbed (as it needs to be for the light to be detected), the pupil appears black. The sclera (in human's thats the white part) can also be colored without any repercussions, and indeed is, in many species.

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