There is a manga by the same author that explains this world a little better.
"Zhen" is the name of the manga, and Zhen are essentially a Harpy equivalent: human head, wings, and talons. Zhen are a species of creature whose feathers are vibrant shades of every color, often a rainbow to look at, and captors/tamers (humans) pride themselves on the way they can manipulate a Zhen's pattern to look more illustrious and captivating based on the diet they feed them. The catch is, a Zhen's natural diet revolves around consuming equally colorful but lethally poisonous (to humans) plants, absorbing and re-purposing the poison for their own defense. A scratch or a bite from a Zhen could be deadly to a human, which is why a very very small number of tamed Zhen will make themselves completely vulnerable and subsequently "white," removing their poison (and color) entirely to intermingle with common people (although rainbow Zhen seem to be worth more, at least in the money sense). The fact that there's a Zhen who's essentially transcended "rainbow" and has become so muddled with color that it's feathers turned "black" should probably hint that to humans, just breathing too close to the black zhen could be lethal. There's a good chance that the white one is keeping the black one as dark as it is both to show others that "look, he's eaten so much poison that his colors aren't vibrant either, so he's not worth a whole lot" and manipulate the situation in his favor to keep the black one to himself.
But i thought the white one have a lover? Why he want the black one T.T
the white zhen of "Zhen" is named Caihong, the white zhen on "Zenniao" is named Liu Xin, so I'm pretty sure they're two different people
Owhhhhhh thanks for clearing up (づ￣ ³￣)づ
I DONT UNDERSTAND HAAAAA (￣∇￣")