The color of the iris is changeable in several ways using a single gradient. The colors of the gradients are passed on to several shaders mixed to achieve the effect.
The shade of the iris and its central corona are controlled independently for a wide range of variations.
Nice green eyes. Dimensions are accurate. All IOR used are from physical measurements. Some of those renders could have used more convergence time, sorry for the noise.
Scary lighting or something. Whatever. To achieve the full translucent effect on the sclera, I modeled the choroid connective tissues inside the eye.
This is pretty much where the illusion breaks. I could use a higher resolution model but this eye is supposed to be shared across multiple characters and I want it to be fairly cheap to save memory. Ideally, I would increase the level of details with displacement. Note the dichromic iris and its reflection in the lens.
Closer to the lens behind the iris. Here, it is showing signs of cataract. Unfortunately, Octane's procedural noises reach their maximal scale limit.
Retina you say? Of course. Flash!
That's pretty much how I tried to model the surfaces inside. It's easy for a glass of whisky but it gets more complex with the inside of an eye. The light has to go through the cornea, then the aqueous humor, then the lens, then the vitreous humor to finally hit the retina. Describing those different bodies inside using the rules you pointed to is a bit of a mind bender.