At the very least Kai'sa should be blue like Kass and Jax

Void exposure messes people up. This has been pretty well established thus far. We don't know how long Kass and Jax were exposed to the Void, but they got some pretty weird mutations as a result. We don't know how deep the changes went since both of them are covered up pretty well, but at minimum it turned 'em blue.

I think that would be a decent baseline for 'exposed to the Void but not subsumed by its will'. You turn blue. And we're notably missing a blue chick since Riot remade Eve.