She does not have read hair.  It's _drawn_ red, that's all.

       *Sniffle*  I feel so deprived now that I never got to read the manga
(except
for the episode involving Ranma & Ryougas reunion where Ranma gives
Ryouga all the bread he stole...and Ryouga still wants to kill him)...and never
got to see the anime.
       Still...all the fanfics Ive been reading say its red...and

Ranma-chan is

a red-head in all my pictures of her...hmmm...maybe she dyes her hair?

it's not dyed..it comes with her female form, whatever color
it is.