I don't believe her hair is actually meant to be purple. Her live action representation is probably a very accurate one. The purple I think was just used to add some colour to her hair and using grey would not really work. Similarly I don't think Sailor Mercury is meant to actually have blue hair either.
Not really sure what's going on with Michiru though...
Well, in a way it is, because of limitations of color on paper. Purple is a logical choice to show a brighter black, but it's still black. Of course now a days with Photoshop, and PSP, it's easier to make black shine, but back before that, different colors were used. Kinda like how a shiney blonde was white.
I think her hair is *meant* to be black, too, but is shown purple/with purple highlights the way Luna is meant to be black but is always colored purple. That being said, I think the full-blown purple hair looks prettier than the black hair. Then again, I'm also partial to Mars having red hair.
It's the same as Rei, except "blue highlights" instead of purple. Sometimes his hair is even a very very dark teal in some artworks, like one of the R2 cover arts with him in his prince endimiyon outfit
I think I'm going to go with her hair matching the color of her bow. I just realized looking at my Volume 3 that came in the mail the other day that her hair is almost a blue/purple with the printing. Actually the whole cover looks a bit more softer, like a watercolor.