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.
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.)
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!
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.
