Sure! Hm.. I don't know many tutorials specifically about that, but I'll tell you what I know.
Ambient light is of course light that effects the figure from the environment it's in, that is not direct light. Imagine all the light reflecting from things like the sky, ground, etc back onto the figure. To do this I lightly brushed warm yellow-tinted strokes with a Screen layer style setting over parts of the creature that would receive the most ambient light.
Transmission/subsurface scattering = light that shrines somewhat through the form. It's what is happening when you see your skin glow reddish when a light is shining through it. Hence why I added a pinkish overlay on parts where the skin is receiving light + whenever it is more translucent. Which is a lot, considering the squid aspect of this guy.
Hey, thanks for the advice! You have saved me many hours lying awake pondering these questions, heh heh.
So, the ambient light involves reflected light from all over the environment, right? From the sky, the ground, close objects, etc? I guess my question is: why did you choose a yellow tint on your screen layer instead of going with colors based on the environment?
For the subsurface scattering, I think I understand - wherever light shines through thin or translucent tissue, the blood will show through. Have you ever tried subsurf with another hue? What if your creature had blue blood, for example?