Glossy Translucent Absorption Color and Volume Absorption Color shouldn’t be the same exact color. The first one is for light bouncing off the skin, the second for light penetrating the skin.
OK, now I’m very confused. If those colors aren’t supposed to be the same, then how do you know what should be what?
According to the information at the Luxrender wiki:
(Volumes) Absorption color: ... The absorption color determines how light is lost as it passes through the volume. This defines an attenuation rate, meaning that the color will be become more dark and saturated the farther it travels through the volume. It also means that this color control will seem to work “backwards”. If you specify the raw absorption color as red, it will remove red light, leaving you with a cyan volume….
(Glossy Translucent) Absorption Color/Depth: ... These allow you to specify the color and depth of light absorbtion by the surface coating. Note that since this is an absorption color, it will seem to work “backwards”. Setting it to blue will cause blue light to be absorbed, leaving you with a yellow-orange appearance. To defeat this option, set the color to full black (0.0). This parameter is not related to volume absorption….
According to that explanation, although they very technically do different things—reflected versus absorbed/passed-through light, although both address it in terms of what gets absorbed—they seem to require the same settings.
I know this is not technically correct, but thinking of it this way helps me visualize what I am trying to do with these settings. “This one has to be set high and this one low, this one is dark and that one is light.” But as I picture it, this is the way it works. Volume is like your muscles beneath your skin. Setting a higher volume scale is like setting the power of your muscle tissue to shine its way through your skin, like when you put a flashlight behind your hand and you can see it shine through. The stronger the flashlight, the more you will see it. So choose a color and strength that will show through the skin (the glossy-translucent settings), but still look natural Don’t choose a red color like your actual muscles, but instead choose something more like a darker version of your actual skin color. And don’t forget to enter it as a reversed color. Red reversed is cyan, Green reversed is magenta, and Blue reversed is yellow. So Caucasian flesh color, which is orangey-pink (red and yellow and white), which reversed would be cyan and blue and black. That means a darker cyan-blue makes a pale peach. I know the documentation says the parameters are not related, but you can definitely see a correlation between the glossy-translucent absorption and volume absorption whenever you change either setting.
The glossy translucent absorption (GTA) parameters are like your actual skin. They represent a coating around the volume. The lower the strength of the GTA, the more of the volume effects will show through. Likewise, if your volume settings are too low, you will end up with a pale, washed out skin. So choose a GTA that approximates the actual color of skin you want and don’t forget to enter the RGB values as opposites of the RGB values of your actual chosen color. As for the GTA scale, smaller values will appear as paler versions of your GTA color because the “skin” is thinner, therefore reflecting more light than it absorbs. Thicker scales will absorb some of the light, bringing out the more true value of the chosen color, while at the same time reflecting light back out through the skin giving it that waxy, translucent look. Find a balance between thickness (scale) and color or you will end up with white tips on the end of the nose and the ears! I think of the scale settings in terms of centimeters, though that is not technically correct. Human skin is, at it’s thickest, about 4mm or nearly 1/2 centimeter thick. So, keep this setting in the .3 - .5 range. A little higher if you are going for an effect.
As for scatter, the scale should be similar to, but not necessarily the same as, volume absorption scale for the same reasons. You want the scattering effect to be visible through the GTA, but not overwhelm it. Here is where I would choose a color with more red in it, since the blood vessels under your skin are flowing with red blood, which contributes to the color of the light scattered under your skin. That said, don’t make it literally red because the tissues in your skin are also contributing to the light color being scattered. I like to choose a reddish brown. This way, you get the redness from the blood, but also the brown of the melanin in our skin. Too much red and you have a sunburned Vicky! If your character is particularly pale, such as a vampire or albino, then you can go for a bluer tone, more likely purple or lavender so that some red is still present. This color does not have to be reversed. It is a direct color because there is no absorption involved. I think of the scale for volume in millimeters (cm for skin, mm for volume). My body is about 17” wide viewed from the front, which is 430 mm. So, I set my volume around there. Guys like the Freak are big, so I went to 500 for him. Females and kids might be around 10” to 14” wide, so 250 to 350 is a good place to start for them.
Again, nothing scientific about most of what I wrote here, it just helps me to think in these terms and seems to work. A few of the stats I listed actually are scientific (at least according to Wikipedia) like skin thickness, etc. These are good starting off points for you, at least. Also note that human skin has an index of refraction (IOR) of 1.45, so set your volume and specularity IOR’s to 1.45.
I hope my theories help someone. I am very sleepy right now and may have rambled a bit and for that, I apologize. Also, thank goodness for spell check, because sleepy people should not be allowed to type.