The functions of the elaborate iridescent coloration and large "train" of peacocks have been the subject of extensive scientific debate. Charles Darwinsuggested they served to attract females, and the showy features of the males had evolved by sexual selection. More recently, Amotz Zahavi proposed in his handicap theory that these features acted as honest signals of the males' fitness, since less fit males would be disadvantaged by the difficulty of surviving with such large and conspicuous structures.
The Indian peacock has iridescent blue and green plumage. The peacock "tail", known as a "train", consists not of tail quill feathers, but highly elongated upper tail coverts. These feathers are marked with eyespots, best seen when a peacock fans his tail. Both sexes of all species have a crest atop the head. The Indian peahen has a mixture of dull grey, brown, and green in her plumage. The female also displays her plumage to ward off female competition or signal danger to her young.
The green peafowl differs from the Indian peafowl in that the male has green and gold plumage with black wings with a sheen of blue. Unlike the Indian peafowl, the green peahen is similar to the male, only having shorter upper tail coverts, a more coppery neck, and overall less iridescence.
The Congo peacock male does not display his covert feathers, but uses his actual tail feathers during courtship displays. These feathers are much shorter than those of the Indian and green species, and the ocelli are much less pronounced. Females of the Indian and African species are dull grey and/or brown.
Chicks of both sexes in all the species are cryptically coloured. They vary between yellow and tawny, usually with patches of darker brown or light tan and "dirty white" ivory.
Occasionally, peafowl appear with white plumage. Although albino peafowl do exist, this is quite rare and almost all white peafowl are not, in fact, albinos: they have a genetic mutation called leucism which causes an overall reduction in pigment which, in peafowl, causes a complete lack of pigment in their plumage, but still leaves them with blue eyes; by contrast, true albino peafowl have a complete lack of melanin and therefore have white plumage, but also an albino's characteristic red or pink eyes. Leucistic peachicks are born yellow and become fully white as they mature.
As with many birds, vibrant iridescent plumage colours are not primarily pigments, but structural coloration. Optical interference Bragg reflections based on regular, periodic nanostructures of the barbules (fiber-like components) of the feathers produce the peacock's colours. Slight changes to the spacing of these barbules result in different colours. Brown feathers are a mixture of red and blue: one colour is created by the periodic structure and the other is created by a Fabry–Pérot interference peak from reflections from the outer and inner boundaries. Such structural coloration causes the iridescence of the peacock's hues interference effects depend on light angle rather than actual pigments.