Animation Source Forums

Nala's eyes were always the same color thorough the first film. Animators were just playing with different lightning and day times on different scenes on the eye color. (For an example, try to compare Can You Feel The Love Tonight scene, Bath scene, scene where Nala first meets adult Simba/the fight with Simba, and the scene at the top of the PrideRock at the end.) While being differently colored in every scene, they still hold that very same teal shadow.

About TLK 2...You are right: it really was an animation mistake by not paying much attention to model.