NPCs around my town started spawning with weird hair colors - usually it's either full blue or black with red, blue or pink roots and tips. None of these are saved as custom hair color presets in my CAS. I'm using a full roster of Nraas mods but no cc or default replacements that could affect hair. As far as I'm aware, there are no stray imaginary friends in my town whose genetics could cause this to happen.
Has this happened to anyone else?

Hi
I'm assuming that you have StoryProgression and that must be one of the reasons for NPCs autogenerating with weird hair colors. By default it increases the chance of auto generated sims to be have more unusual look in intend to increase the town's population variety.
You could also share your mod list in case you don't have StoryProgression.

Edit: I also have some autogenerated NPCs around Bridgeport that have weird features, but in my case I use Pudding Face fix by Lazy Duchess. This mod only manages the look of newly generated townies unlike StoryProgression (these two mods should not be used together).