Not one to dispute the master, but I do have personal experience that makes me question these theories.
First, my own eyes slowly oscillate between green and blue. When I say slowly, I mean over a period of several days. It doesn’t seem likely that the fat content of my eyes would be changing.
Second, and even more dramatic, I had a girlfriend who had, what I used to call, mood eyes. Her eye color could change from brown to green to blue, all in a matter of minutes. She would also get this eerie (cat-like) gold color from time to time. I find it very difficult to believe that the melanin and fat content of her eyes could change dramatically so quickly. There must be some other explanation…
Of course, Cecil alluded to a more detailed explanation, but didn’t deliver.