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An interesting aspect of hair rendering is that all the intricate visual details seen in hair stem from the scattering of light in a multitude of hair strands and is not due to the complexity of the BRDF on a single hair strand. This seemingly simple observation explains why so many complex shaders are not successful at conveying a realistic look for hair. This includes the widely used shaders based on the Marschner model not using volume scattering. Our shader simulates 3 scattering events from one single hair strand and then proceeds with a Monte-Carlo simulation to follow light paths inside the hair volume.
Two observations:
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Note that it offers no control over directly reflected color. Direct reflections have the same color as the incoming light. Any coloured visual features are due to light passing through a strand of hair.