Board Thread:Speculation House/@comment-96.236.119.41-20161215012520/@comment-25936766-20161215022342

The simplest answer so far is that they become Red purely because that's the activation cue of her Semblance, and specifically Red due to genetics. Most other explanations are little more than hypothesis at the moment.

One thing I note is that....her Eyes become Red when she gets really angry. Although she tends to activate her Semblance whenever she's really angry, which could have mislead us.

It could be her eyes only get Red when she's really angry, being irrelevant to her Semblance. This hypothesis can be extended to other "Eyeshifters", if we assume the eyes change color based on emotional state.

With Klein, they changed color as his personality changed though, instead of mood. And with Tyrian it's hard to say if his mood really changed (he didn't seem any different before and after his eyes got purple, and I say that in both a negative and positive sense). And with Neo it's impossible to know if the eye color changes are purely because Semblance or not.