The question is not "what color is the wall?" but "what color are you seeing?". So what color are you seeing when looking at the red wall in perfect darkness? Better yet, are you seeing anything at all? Are you claiming that anything you can't see, on the other side of the wall, is also
black because you can't perceive any spectral emissions from their surface?
Earlier you said the wall should be black because black is a color and that's what you'd be seeing. But now you say the actual color is red but only using prior knowledge. And if the wall actually changed colors (I quickly painted it blue just to prove you wrong) what color would you say it is? Black? Red? Blue? So your example is purely philosophical and doesn't support your claim one bit.
Is the absence of light a type of light? Can we call the absence of any photon a "close04-ray" in honor of it's distinguished inventor, me?
A room doesn't contain void, it simply lacks everything else. Water is odor-less and taste-less. Silence isn't a type of sound, it's the lack of sound. And a completely empty fridge doesn't contain food. Your definition of a color while correct sabotages your conclusion that "detecting no spectral emissions" should be called the color black.
And please stop using additive/subtractive as if this should change how colors are perceived. It changes how they are produced but your eye will still detect light at a specific wavelength that you'll call a color.
Colloquially people will still call it the color black. But it's just a convention also meant to support the fact that we call a wide range of greys and dark colors as "black". It's for ease of speech and description, not for accuracy. In reality true black is simply the absence of any perceived light.