This distinctly reminded me of the old series Star Trek episode Let That Be Your Last Battlefield, which was a slightly heavy-handed take on race. It features two humanoid aliens with such a bilateral black-white split - but who are implacably opposed because they differ in which sides are black and white.
And this in turn jogged my memory about an SF story I read decades ago, Theodore Sturgeon's The Comedian's Children, which features a viral syndrome affecting children, which causes one-sided paralysis and a pigmentation change:
The most spectacular symptom was on the superficial pigmentation. The immobilized side turned white as bleached bone, the other increasingly dark, beginning with a reddening and slowly going through the red-browns to a chocolate in the later stages. The division was exactly on the median line, and the bicoloration proceeded the same way in all cases, regardless of the original pigmentation.pages 88-128, which I was pleased to find in the Internet Archive's Pulp Magazine Archive (I must explore that more). The Comedian's Childen hasn't dated much, and remains a very sharp take on the conflict between medical and celebrity figures in the area of high-profile children's diseases. The fictional syndrome is called iapetitis, named after Iapetus, the bi-coloured moon of Saturn that features in the story.
- The Comedian's Children