I read (on r.a.a back when it was r.a.a and I read it) that
onna-Ranma's hair was originally black, but Takahashi saw the anime
and decided that she liked having Ranma's hair change color, so from
some point in the manga onna-Ranma's hair is canonically red.
(I personally don't like having onna-Ranma's hair be a different color
than otoko-Ranma's, as that rather counters the effect of onna-Ranma
and otoko-Ranma looking so much alike. At worst I would think that
onna-Ranma's hair would be about the same color as Takaya Noriko's)
Of course, I have also heard that Takahashi fired a large
portion of the anime production staff after the first 18 episodes of
the anime. All I can say for sure is that the character designs
definately changed.
hope it helps,
Suika