Ranma's hair changes color in the anime because it looks nicer!  It 
doesn't change color in the comics because black is easier to draw! (I'm 
a comic book artist too, so I know these things)  The hair on the covers 
must be for the people who saw the anime and want to buy the manga.  This 
makes it more recognizable to them.  Having a main character with red 
hair in a black and white book is a no-no for many reasons:
1.  You have to draw the character many many many thousands of times, so 
having it fill in his/her hair with screen tones would be pestering.

Just like Ryoga's do-rag...perfect squares every time...
i don't think it's _that_ troublesome.

3.  Which screen tone is right for red hair?

i think a half-tone (gray) would be right.

by the way, have you noticed that on Viz's posters, Ranma-chan
has black hair?  Just something I noticed...doesn't mean anything,
of course, Viz of all groups would get it wrong ^^

signed, phoneynt