As a silkie breeder, I find this very interesting. In processing and eating my culls I have noticed that the depth of color of the flesh and bones of the silkies varies- between varieties and individuals. An Asian friend I occasionally give butchered birds to prefers the white birds.
Now, you seem to indicate that the insectivorous diet is necessary for the dark mutants to survive. Do you believe the diet affects the depth of skin color?
By the way, thanks for reminding me of the word 'phytoestrogens'. I was trying to think of this word the other day when trying to explain why I use gamebird feed to condition birds for breeding and through breeding season- only feed I can get here with animal protein without mixing my own.