This is an interesting question – though I can not contribute to its solution.
I can only say that this feature can not be seen in all Paro species ………… e.g, I have never seen it in P. quindecim. Their eye rim is always pearly white. I dare to say that in female quindecim it helps to distinguish them from females of other species.
So I think that the colour and maybe moody colour variations of the eye rim might contribute to distinguishing different species/forms.