We do have four sectors, 3 color perception and then the brightness perception that is used in the dark. In mid darkness you get a mix of all of those, although the fourth is not really perceived as a color so it can be a bit hard to use.
replyBrightness is another dimension, not a "sector" (as I dubbed it) on the color spectrum. But it would be equal for all subjects in a test, so it can't add information.
replySomething I think about often is an oliver sacks book about an ethnic group that has a particularly high rate of true monochromacy. And the people with no color perception at all are particularly adept at spotting certain plants based on some characteristic of their leaves that is obscured by color. So even removing information can change perception in surprising ways.
replyOTOH sacks seems to have fabricated a lot of shit over the years so who knows if this is even real. Another thing I think about a lot now.