To change the color of eyes of "Sims" already in the game you should use a cheat to edit the Sim in CAS
First: ctrl + shift + C then then write testingcheats true and after that type CAS.fulleditmode
Now with the "shift" pressed click on the Sim and choose edit it in CAS.
Attention: When you enter the CAS, the whole family is available for editing, but NOT edit more than one sim at a time, your game will give error
I hope you enjoy! =)
If you prefer other colors say in the comments, I can add more colors.
Kuree for The Sims 4 Studio