Well, I used 0.5 as a convenient and intuitive midpoint of the 0-1 luminance range, but this of course is a simplification and doesn't align with human perception (edit: it is aligned), it was more of an example if anything.
You are right, 0.18 is indeed perceptually closer to "middle gray" because the eye responds more sensitively to darker tones, so yeah, using a threshold closer to 0.18 makes more sense if we want to identify whether a color "feels" light or dark.
That said, 0.5 is a mathematical midpoint, but as I said, not aligned with how humans perceive brightness (edit: it is aligned).
You are right, 0.18 is indeed perceptually closer to "middle gray" because the eye responds more sensitively to darker tones, so yeah, using a threshold closer to 0.18 makes more sense if we want to identify whether a color "feels" light or dark.
That said, 0.5 is a mathematical midpoint, but as I said, not aligned with how humans perceive brightness (edit: it is aligned).
Ultimately one could use 0.18-0.3 as threshold.