Okay, this is a very good question.  Many people don't spot it right off, so
here is a simple test you can do that will show you how it looks and what it
does.
    Do you own a red laser pointer?  Using a dark card, go in a darkened room
and place the card against your skin, edgewise.  Now, shine the laser pointer on
the skin on the other side of the card- not the part of the skin you are looking
at.
    When you see the red, diffuse glow under the card, you are seeing subsurface
scattering.  But you probably knew that.  In looking at the surface of, say, a
mannequin, you will see that the shadows are very hard and there is no
subsurface scattering.  With real flesh, there is considerable softening of the
shadows and a really significant amount of light diffusion beneath the surface.
    Skin is actually transparent to a number of frequencies, but in visible
light, it tends to be right in the red end of the spectrum.  If you model skin
with an internal red scattering, or a red diffuse medium, it looks far more
natural when light falls on it.  Instead of that blocky, plastic look, it has
the depth and softness that we expect.
    Seeing some of your renders in this group, I know that you are aware of how
the little details make or break an image.  This is one of those things that
really makes people look realistic.
    Oh, I tend to use a pale orange sort of color when playing with skin
subsurface scattering.  Use white light for the experiment above and you will
see that this is the dominant color that appears to the human eye.

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

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