The blender hair directionality problem.

Originally, I thought this were a terrible bug in blender. That hair, when pointing "up", renders with its correct color, but while pointing "down", shows up as black.

Here are two different versions of the same scene, a character with white hair. The only difference is that the "AccZ" component of the hair physics is changed from +0.01 to -0.01. The three views from left to right are a screen grab of the 3D View in blender, a "Render This Window" (see the icon at the far right of your 3D View buttons), and a true render of the scene.

While the first two images are incorrectly textured, the third one is not. Cause for alarm? Eh, while it's surely annoying, the final render I guess is what ultimately matters. Unless you're sharing previews of your scenes with your audience.