so... you're asking what the lack of color is, if it isn't black? physically, "black" is the lack of color, ie darkness, which just means that there's no reflective light bouncing off a surface towards your eyes (within the wavelengths of light you can see). what you see is always reflected light bouncing off or coming through material.
link. some blacks are blacker than other blacks, and material can never absorb all light, even superblacks like
vantablack. it's never as dark as empty space, where nothing is reflected. that's the physical part.
then there's human experience. someone else said it, but: while eyelids are near opaque, they're slightly translucent. as you point out, what's seen is a degree of a reddish brown, varying from very dark to very
very dark depending on the surrounding light. the difference is so subtle that it's
experienced as darkness/black. (i don't know about valka's situation.) you wanted the physics of this, not the psychology, but you can't have it just one way when discussing human experience. the physics of experience is often within the field of psychology, overlapping with some human biology naturally. there's a bunch of quirks in human experience that have physical foundations for its explanation but has to do with our brain interpreting the world to simplify it. for example, all humans have a tiny blind spot in our vision that our brain removes to make every day more manageable (here's a
link, noting that a blind spot is normal). "physics" can explain the blind spot, since it's there because a part of our eye receptors are blocked internally in the eye, but they aren't sufficient for explaining where the color in between "comes from", since it's just the brain lying to you. there's not a "physics" explanation for this light, as it's not externally bouncing light, but your brain soup going "let's simplify this". so you can't just use physics for that.
