Originally Posted by amphirion
what causes it then? would you happen to know?
Not sure. Never found any literature about it. But since it was never discussed in genetics, I thus assume it has nothing to do with color genetics.
Physically dark shades of irids can be assumed to have black genes. But I often had light shades of irids which still had black heads . . . . sometimes the heads are more brown than black. IBC, who classifies colors and patterns with scrutiny, never fusses about dark heads.
atb - the picture looks as if the male is turquoise while the female is royal blue. Therefore you should have all irid colors, more turquoise than the other two.