What has the color to do with it? o . O Nyotengu and Kokoro have the best hair animation in the game and they are pretty close to black. It's more a problem of length, polygon count and collision programming.
That's exactly what I thought, too.
I honestly have no idea, and I just thought it was all just about the mesh, bump maps, and UV mapping in terms of materials. I do remember reading numerous articles from Square Enix stating that they cut Tifa's hair down because the longer black hair was very difficult to animate, as opposed to other loose hair in other long-haired characters like Sephiroth, though, then again, that was back in 2004-2005, so obviously it's more advanced and maybe easier now. You are right, though, and maybe they just find it a very tedious long eight hours a day to do, as opposed to the shorter long-haired characters, considering Momiji's hair is the longest just falling from her thigh.
Then again, I have no idea, because it's probably just the amount of strands they have to make (or copy-paste from other characters, which doesn't seem hard to do).
I don't get why they wouldn't do Momiji though, in terms of loose hair.