There is a trend in teenage fan-fiction fiction for creating characters with overly interesting eyes. Either they're a completely unnatural shade, cat-shaped or perhaps bioluminescent, but the one I find truly groan worthy is eyes that change colour. Double demerits if the eyes change colour according to the character's mood. Googleplexian-demerits if they're the only character in the world/book with emoti-coloured eyes.

I take comfort in the fact that I'm not the only one, here - somewhere along the way, enough people have noticed this that its made its way into various Mary-Sue litmus tests. I encountered an otherwise-well-written novel the other day whose main character hit the jackpot - the only eyes in the world that flashed gold whenever she was happy, angry, excited, surprised or presumably any emotion other than morose.

On the basis of it being otherwise an enjoyable read and a debut novel, I granted the author the benefit of the doubt and grimaced my way through each passage that was devoted to those damned irises. But I started to wonder - exactly what is it about eyes that change colour that makes them so (forgive the pun) eye-roll worthy?

We have characters who change their skin colours to communicate, whose hair changes colour under stress, whose teeth lengthen and who (of all things) sparkle whenever the UV gets too high. Why do we draw the line at eyeballs? And why the hell is it so common?

The argument against colour-change eyes is quite simple:

• someone's eyes changing colour is a far more noticeable (though much less nuanced or subtle) expression of their emotion than the set of their mouth or their brows. 
• in writing, if something isn't written, it hasn't happened. If Sarah's eyes change colour from green to red and you don't write about it, it didn't happen.
• the reader only wants to be told something once. When showing someone is angry, you don't say their shoulders were tense and their foot was tapping and their arms were crossed and they were scowling. You pick one thing.
• therefore you have the dilemma of a character whose eyes change according to their mood only sometimes, because other times you chose to describe something else, or a character whose moods are only described by the colour of their eyes. 

And, most importantly: every time you tell us about the character's eyes, that's all we get. You're not showing us what the character feels, you're just giving a shorthand. There's no nuance to their feeling, it's just a placeholder for an adjective. You don't see the tremble of the lip belying fierce glare, or the difference between fear, horror, terror, shock or panic.

There's nothing to pull us into the character's head - we can't emphathise or relate to someone's emotions by the fact that their eyes changed colour, because ours don't. We relate to someone throwing something at the wall, or biting their lip - ways that we express emotion. Someone's eyes changing colour is not a human expression of emotion, so it conveys much less information in place of something that would have connected with the reader.

The other issue is that it very rarely has a real purpose - most of the time, it's purely a mechanism to make the character 'special' or 'the chosen one'. It doesn't serve the story in anyway - they could have been just as special and chosen if they had a birthmark in the shape of a peanut on their cheek. But that wouldn't be as glamorous. 

Chameleon skin and hair have, in theory, a potential purpose in evolution as a form of communication - look up cuttlefish, for starters. The body or hair is a large enough surface that complex shifting patterns can indicate a wealth of information. In a speculative fiction setting, evolution could easily have gifted the character's species with these gifts. Even if your character is the only member of their species surviving, you can 'get away' with such devices, provided they fulfil the story in some way. (If they're just there for decoration, or 'character-specialness', get rid of them now, before you write another word.)

Eyes are too small, too well-hidden to be useful chameleonic communication devices. You just can't get enough info on there for it to be useful (and if you're trying to, your reader is not going to bother remembering which of the 47 shades of blue means 'angry but in a kind of sad way with a little bit of guilt'). So, we have no reasonable process for it to have evolved, no acceptable reason for it to be there (to fulfil a prophecy is not an acceptable reason - see above re: peanut-cheek) and we're left with it only being there because the writer thought it would be cool.

Typically, because the writer thinks it would be really cool if they had colour-change eyes. This is why it's up there in the Mary-Sue test - something is serving the author instead of the story. And it's obvious - that's why it makes us cringe. The author's wish-fulfilment is flapping about in the breeze, and taking the author's dignity with it.

Same goes for anything in your story: if it's only there because it's cool, then it shouldn't be there. Write the story, not yourself.