A seemingly simple question that will most likely send you down a rabbit hole. The answer(s) depend on several things:

Which version of Poser?

Firefly or Superfly?

What's your scene and lighting?

How are the characters eye materials setup?

Poser version may be important - or may not - but it's best to give as much info as possible.

Firefly and Superfly treat materials differently.

There are some instances when, even with realistic lighting, eyes may appear to be white.

AFAIK, if you're using the default material setup you won't learn anything except how to cry. SnarlyGribbly has developed shader systems for Firefly and Superfly, called EZSkin and (I think) EZSkin 2. They generally take care of setting up stuff so that SSS works properly. If you don't have them, you should grab them ASAP. Can't remember the site but Google - as usual - is your friend there.

Coppula eam se non posit acceptera jocularum.

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