What's the word for the type of gray when the lights are turned off?

Eigengrau (“intrinsic gray” / “own grey”), also called Eigenlicht (“intrinsic light”), dark light, or brain gray, is a term for the uniform dark gray background that many people report seeing in the absence of light. It is the dark grey colour seen by the eyes in perfect darkness, as a result of signals from the optic nerves.

Even in the absence of light, some action potentials are still sent along the optic nerve, causing the sensation of a uniform dark gray color. Eigengrau is perceived as lighter than a black object in normal lighting conditions, because contrast is more important to the visual system than absolute brightness.

Hence, what we see in the dark is not black (000000), but the color eigengrau (16161D hex or 22,22,29 RGB).