[center]Place: Bannarghata biological park, Bangalore[/center]
[center]Date: 19/9/2019[/center]
What condition do you think caused this peacock to have an all white plumage?
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Beautiful indeed 
The white feathers on a peacock depend on which genes are dominant and which are recessive. They don’t have any pigmentation. Any animal can be albino, but what separates an albino from a white peacock is the colour of the skin beneath its feathers . A white peacock has coloured skin whereas an albino’s skin will be very pale and without any markings.
According to United Peafowl Association Knowledge Base, the first color variation was the blackshoulder, which appeared in 1830. When the white first appeared is unknown. White, pied and blackshoulder color patterns of Indian blues are not often found in the wild. Any patches of white would make the bird more visible to predators. The theory is that in the safety of being in captivity, their recessive coloration genes have emerged. This has also happened in pet birds such as the zebra finch and other animals like the gerbil and the Syrian hamster.
This peacock is white because of a condition called Leucism.
Leucism is a genetic mutation that causes loss of pigmentation. Animals with leucism retain their normal eye color. It’s the feathers that do not have pigmentation. White feathers depend on which genes are dominant and which are recessive. White Peacocks are not found in the wild. Patches of white would make the Peacocks more visible to predators. The theory is that the recessive white coloration genes emerge in animals kept in captivity. This sometimes occurs with pet birds or animals. This gene mutation explains the existence of white peafowls.
There is a variety of white color patterns that have appeared in peafowl bred in captivity:
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Pied White – Combination of white and usual Indian blue colors.
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Black shoulder pied- Coloration is normal except for white under parts, wings and a spot under the chin.
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Black Shoulder Peahen- White, sprinkled with black spots.
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Yes. The eyes definitely give it away that this isn’t albinism. 
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It has melanin pigmentation of one colour in it’s barbule. The colour is due to metasurface and melanin. Are these peacocks white due to loss of melanin or change in the metasurface?
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These peacocks are white due to loss of melanin in the barbules. So only the outward plumage is affected.
What do you mean by “colour is due to metasurface”?
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The PNAS link provides all the detail.
It has melanin pigmentation of one colour in it’s barbule. Consequently peacock feathers should have only one colour - brown - not the multicolour display we see.
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“The cortex of all differently colored barbules contains a 2D photonic-crystal structure made up of melanin rods connected by keratin. Melanin is created by melanocyte cells, deposited in developing feathers, and becomes fixed in the keratinized feather structure. Photonic-crystal structures in all differently colored barbules are quite similar. In the blue, green, and yellow barbules, the lattice structure is nearly square, whereas in the brown barbule it is a rectangular lattice. The only differences are the lattice constant (rod spacing) and the number of periods (melanin rod layers) along the direction normal to the cortex surface.”
So doesn’t this article imply that the peacock feathers have the multicoloured barbules and not just brown?
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I think what @jtd was implying that the barbules aren’t multicolored due to melanin. Melanin makes the barbules brown. Melanin gives brown color shades not blue or green or yellow.
The color of barbules that the paper is referring to is the one that is emergent property of photonic crystals not the actual color of the barbules which is brown as pointed out by @jtd due to melanin.
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So leucism could also be due to the lack of phototonic crystals?
Or is the lack of melanin the only factor? 
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LAck of melanin.
The barbules are not multicolor due to pigment colour.
However despite the photonic lattice - spacing and distribution of mechanical features made up of the same material - remaining the same, lack of brown pigment makes the feather white!.
Hence a single statement that eliminates comment on either the photonic structure or pigment would be ambiguous.
Wonder what colour would emerge if the pigmentation was changed to a different color.
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It depends if leucism is caused on animals with pigmentation or with coloring due to photonic crystals. Don’t get this wrong, there are other pigments than melanin who give a wide range of colors ranging from the rarest blue to the most common shades of brown. Therefore coloring can be due to photonic crystals or pigmentation.
Now when you talk about loss of coloring the factors it depends on are the factors that caused the coloring.
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