UPDATE 26JAN21: I have added a new version of these eyes which have a more pronounced shadow from the upper lid, and a total of 32 colours. Do not install more than one package or they will conflict!
“Central heterochromia is when the inner ring of the iris — the eye color closest to your pupil — is a different color than the outer ring, along the edge of your iris. This trait usually involves both eyes, with two separate colors appearing in each eye instead of one. Central heterochromia is almost always harmless when you’re born with it.
The visual appearance of central heterochromia comes down to a pigment called melanin — the same pigment that determines the color of your skin.
When melanin is distributed differently closer to the pupils, light reflects off of it in a different way and gives the appearance of two different colors in each iris.
The Sims 4 EYE COLOUR – Central Heterochromia
This upload consists of 23 colours of central heterochromia eyes. They are in the “eye” category in CAS, and are for all genders/all ages/all outfit and archetype categories, Human, Mermaid, and Vampire, and tagged by colour as best I could. The files were created as an Add-On set of swatches, so they are not default replacements, merely an addition.