Eye system

"You see kid, scientists had to come up with a way of being able to identify serious criminals. With powers it became too easy to cover up a murder. People used to be injected with a special substance the moment they were born which stayed in a part of their head for the rest of our lives, but it was so long ago, our brains evolved and started to create the substance by itself. Right now, under certain circumstances, certain emotions trigger the fluid in your brain when you're convinced you have committed manslaughter, which causes your eyes to change color. Still, it's a faulty system, since people might think they committed a crime which could've just been a hallucination or-"

"-Yellow eyes mean you killed someone in self defense, orange eyes means you were heavily provoked to kill, red eyes means you killed someone for sport. That's it."

-Avery and Yuki on the eye system

Eye system
The eye system is what scientists came up with to identify serious criminals, because you could cover up murders too easily with powers. Society used to be injected with a special substance when they were born, but it happened such a long time ago their brains evolved and started producing it themselves. Currently, under certain circumstances, i.e when you believe you've committed murder, some emotions will trigger the substance and cause your eye colour to change. It's still a bit of a faulty system since you might not have actually murdered someone and were hallucinating.

The eye system is taught in 6th grade.

Colours
The chemical that your brain releases when you've murdered someone can emit 3 colours into your iris. Yellow, orange and red.

Yellow means you've killed someone in self defence.

Orange means you were heavily provoked to kill.

Red means you killed for sport.

Characters
So far, we only know 3 characters with colored eyes. Alice Hart possesses yellow eyes. Another character we know of that has colored eyes is 13-year-old Lydia Florise (niece of the king), who has red eyes after being accused of murdering 43 people, including her father. 95% of victims were associated with the movement of banning hybrids. She was later executed. The last one is the leader of 'HES' (the Hybrid Extermination Squad), Named Jacob Forris. He possesses orange eyes, and does not wish to follow the orders of the king, which is to keep his people from killing off more Hybrids. And says that if a single crime involving Hybrids appears, then he won't hesitate to put his people back to work.