I've been thinking about this some. Let's say you have a really accurate skin texture that is highly photorealistic. The areas that are closer to bone (like the forehead) are more "yellow."  (IOW yellow undertones). Then you have blue veins that pop out. 

So, the color yellow is a mixture of R and G in the RGB color spectrum. The red values in the yellow color would be multiplied by the R depth, and the green value would be multiplied by the G depth.

And the Blue values in the veins would not scatter as deeply as the flesh colors.

 At least that's my understanding.But I pinged Nerd to jump in on this