I've never heard of using eye color to sex fish until I read Lee's article. He said that sexually active male syns all have blue color, and females yellow, so apparently it only applies to mature specimens. On closer look of my Viejas collection, my male Syns, male Melanurum and a blue male Fenestratus all have blue eyes. The only male Vieja that has yellow eyes is my pink/white Fenestratus which may have to do with its lacking of pigmentation. I have two immature V. bifas, one has blue eyes and the other yellow. Time will tell if their eye color will determine their sex.