Ideally hair has three tonal values being Highlight, Lowlight, and Midtone. As sometimes done with skinmap, you could have 3 maps each defining the hue, color, and saturation for each.

http://www.for-the-birds.net/Hair.jpg

You can see how noise can simulate cuticle on the hairshaft. It is especially important in the speculat highlight(not the color highlight).

As I mentioned in the IBL thread, notice how color becomes more saturated when lit and desaturated in shadow. A common 3d mistake is to make lit surfaces lighter and paler.

-Anton, creator of Apollo Maximus
"Conviction without truth is denial; Denial in the face of truth is concealment."

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