I would guess its enzyme related to the morph. (not caused by uvb or other external stimulants) This color in in the sclera of the eye and is common with many animals.
I have made and interesting observation. Bearded dragons don't have an Iris.
They have a sclera and a scleral ring. Basically, they have the whites of their eyes and a pupil. There is no Iris. (This is where color should develop.) Bearded dragons don't have iris's. Thats why we see white with vascular activity. (no color, blue, green, brown)
See my new post soon.