I recently rewatched all of Avatar: The Last Airbender and Legend of Korra, and I found a pattern. All waterbenders have blue eyes, airbenders have grey, firebenders have hazel and earthbenders have green.
Has this been noticed by anyone else in the Avatar universe? If so, how was Team Avatar able to stay unnoticed by so many in the Fire Nation if they couldn't change their eye color?
I do not think what we see in the show provides any evidence that there is any causal relationship between bending power and eye color, beyond tribal/familial/genetic ties.
We see many people who possess blue eyes, but not all are Waterbenders. Sokka is one prime example of this. In fact, the lack of reaction to Katara's blue eyes while she was journeying in the Fire Nation with Aang is actually evidence that blue eyes are not a mark of a Waterbender. She claimed to be a refuge from the colonies, and no one really thought anything of it.
In addition, in The Legend of Korra, we see several "mixed race" benders, and they do not support this "eye color causation" theory.
Tenzin has blue eyes, like his mother, Katara, but he is an Airbender.
Tenzin's children also display deviance from the "Airbenders have grey eyes" theory. Jinora, Tenzin's daughter, is an Airbender, yet she has brownish eyes.
Tenzin's newborn son, Rohan, has bright green eyes. He has not been shown Airbending as of yet, but Katara predicted that he would become the most powerful Airbender of the family.
I believe when it comes down to it, this is really just a confusion of correlation and causation.
Kind of.
The novel The Rise of Kyoshi discusses the topic a few times.
Every piece of grief he got from her was made worse by the fact that she resembled her daughter Rangi so closely, with the same porcelain-doll face, pitch-black hair, and eyes tending toward darker bronze than the usual Fire Nation gold.
“Respectfully, I gained my position through knowing facts beyond what you think a typical blue-eyed southern rustic should know,” Tagaka snapped.
She hadn’t noticed his eye color, but then, not every Firebender had blatantly gold irises.
As does the The Shadow of Kyoshi:
Over five hundred pairs of golden eyes stared at the Avatar and her companions... It seemed like the entire assembled nobility of the Fire Nation was present
She snatched away the cloth tied over his eyes to reveal golden irises. He was Fire Nation, though he looked very much like the man he was impersonating
From these quotes, we can gleam that expectations and stereotypes exist. However, exceptions are not unheard of.
This is a well known genetic phenomenon, known as gene linkage. It means the genes for eye color and bending are located close to one another on the same chromosome. This means they are rarely separated. Coupled with the colloquial genetics idea (Earth Kingdom people tend to have green eyes, etc.) this makes sense.
So we can conclude that the genes for Earth and Water bending lay on the short arm of Chromosome 19, close to 19p13.1-q13.11, and the genes for Fire (brown) and Water (blue) bending lay on the long arm of Chromosome 15.
Air bending is likely entirely different, considering all air nomads were air benders, but Aang's descendants are not pure-breeding air benders. It's likely on a completely different chromosome, and the entire population of Air Nomads were from a common ancestor or population. It's also not linked to any eye color, because look at Jinora and Ikki. This could, however, be explained by the two-"Not Brown" gene loci, meaning they could have inherited the Chromosome 19 "Not Green" gene, but gotten the Chromosome 15 "Yes Brown" gene from Pema. It is also likely Pema could be carrying the Chromosome 19 "not green" gene, as the Chr15 "Yes Brown" gene is dominant over the Chr19 gene.
Rohan could be explained by Pema carrying one copy of the "Yes Green" and one "Not Green" gene and one copy of the "Yes Brown" and one copy of the "Not Brown" genes.
So air bending could be on Chromosome 19, or it could be on any other autosome other than 15.
My theory is that all waterbenders have blue eyes and all firebenders have hazel/brown eyes no matter what genes. Ive also rewatched tlok and realized that some earthbenders have brown eyes which could be gene related and airbenders can have any depending on genes also. But I believe its more based around bending rather than genes. I know my answer isnt that scientific it just what i observed
