Hey everyone! I re-found this webpage:
http://nala.ru/nalas_eyes.htm
It explains how Nala's eyes didn't really change colors, they were just affected moreso than others during lighting changes in the film.
An interesting article written by Tailbrush, formerly known as Sarafina in the TLKFAA.
( http://fanart.lionking.org/Artists/Tailbrush/ )