This makes four heterochromic Kuronekos on Danbooru now. Can anyone explain what that's about?

Artist probably just likes heterochromia.

It's the same color combo from four different artists, and not all by the same uploader.

It seems to have something to do with the events of Volume 8 of the novel. The relevant Pixiv tag is 神猫.

glasnost said:
It seems to have something to do with the events of Volume 8 of the novel. The relevant Pixiv tag is 神猫.

just to make it clear, those kanji read Kamineko, Kuroneko's chosen new nickname in volume eight after she changed her outfit to the one in this image.