I'm not of the opinion that this is s direct signalling mechanism anymore. I can't speak for 20 years ago, but I think now unnatural hair colors are much more common because they are more widely accepted by society. Where, previously only punks and queers had unnatural hair colors, now, depending on the are, its almost mainstream. I think they only real connection to support for human rights is the correlation that socially conservative folks tend to be less likely to follow new fashion trends then socially liberal and left wing folks.
Discussion
Regardless, women with unusual hair colors are using their appearance to signal which aspect of society they aren't conforming to. My point is that it started out subtle by "changing to a redhead", but most of the time their "redheadedness" didnt mix well with their natural haircolor so it simply looked unnatural. Then the unnatural look became fashionable among the female nonconformists, and it is now extended to highly noticable colors.
For men not interested in being bound and tortured by a woman for eternity this is important to note, because its a very obvious "red flag" (pun intended) of mental illness to avoid.