https://sims4studio.com/thread/720/make-standalone-eye-recolor
Introduction
This tutorial will show you how to make a standalone eye recolor. The swatch thumbnail for it will appear in the eye color swatch thumbnail list just as EA's eye colors do.
What you will need
Sims 4 Studio
Instructions
Using the .dds you exported as your template make your new eye color and save it.
Click on the swatch thumbnail and select a color for your new swatch thumbnail. Note: the eye system used in Sims 4 does not allow custom catalog thumbnails for eye recolors.
Put the .package in your Mods folder and check it in the game. Your swatch will appear in the Eyes swatch thumbnail list.