The pink is simply blood showing through a relatively thin layer of skin epidermis. After a bird gets older, the skin gets heavier scaling, so it is thicker and the pink does not show through as much - if at all. Younger birds are apt to be more active in the breeding pen, so there may be some truth to the claim, but there is no direct relationship between fertility and thin skin per se.
Anyway, that's my take on it. Speaking of thin skin, did anyone else read Paul Smith's article in the April Poultry Press? I had a good laugh over it.