The wiki entry for the http://www.creationkit.com/SetEyeTexture_-_Actor function says that "This will only last until the actor reloads their 3D".

What's meant with that? More specifically, when does an actor reload their 3D?

Much of the data required to play the game is stored in the save. Often when you start up Skyrim, instead of loading data from the BSA files, the data is loaded from your save game. If and when the game reloads a cell from scratch, like after a cell reset, actors in that cell with load their 3D data from the game and not from the save. What that bit above is telling you is that the eye texture change won't be permanent. It will last until such time as the game access the original data from its own files as opposed to your save. There are ways around this, though.

That explained it. Thanks.