Real color perception variance is probably similar to visual sensitivity to stimuli, which, in my experience, covers about a magnitude and is partially due to differences in visual acuity. As is the case with stellar images, defocusing often makes colors more apparent. Since doing so spreads the light out, reducing unit intensity, size appears to be a bigger factor than intensity. As an aside, the small % of women (only women) who have tetrachromatic vision (4 different color receptors) would see colors differently than the rest of us, but, since they use the same language, we'd never know how.
Only a quick reply (the remainder tomorrow, I have a lot of work to do). One full magnitude seems a little bit to me (provided people have normal vision not cataracts etc). We often test the limiting magnitude and the difference are 0.2-0.4. Anyway, if we accept 1 magnitude, taht means that if the average limit is 17.5, then the best might be 18.5. This seems to me very extreme (but maybe maybe maybe...). One proof si the full moon sky: it is blue (close to maximum scotopic sensitivity) and shines at 18. It is big enough not to pose any "size" problem. I have not yet found any people who sees its color. There is a nice written by Brian Skiff on this http://www.astropix.com/HTML/L_STORY/SKYBRITE.HTM
"Could the dark night sky background be seen as blue from a dark sky site?
Unfortunately the moonless dark night sky simply isn't this color---there's practically no emission blueward of the 5577A line (which in daylight appears a lime- or chartreuse-green), only scattered Sunlight (from the zodiacal cloud, which dominates even at the ecliptic poles). Typical photometric B-V color of the sky is 0.9, slightly on the yellow side of white (the Moon has B-V of about 0.9).
The night sky at Full Moon (about magnitude 18 per square arcsecond or about magnitude 9 per square arcminute) must be very close to the color threshold for at least some people to see the night sky as "sky blue".
Several times I've got impressions that it seems blue, but haven't been able to convince myself. Probably just a little brighter would do it. I think the canonical "textbook" figure is magnitude 15 per square arcsecond as the color threshold, which really isn't all that much different from the Full Moon level, so the "slightly brighter than magnitude 18" value doesn't seem unreasonable to me."
As for the "size" problem i think the two factors do not interact linearly. As long as you stay above cones thresholds probably you are rtight: size is more importanta (that is the same for shape detection). However as the luminace approaches the threshold it becomes suddely the limiting factor.
For what concerns the tetracromatic vision of woman i read that they do have 4 types of cones, but the two type seems lumped together and the remaining of the processing uses r/g and b/y opponet mechanism. It has been evaluated that a full 4-dimensional color space perception woul need an addition opponet channel and the whol optiucal nerve and visual cortex should be "re-engineered" for that. The rteason why the color space is 3-dimensional is that it is probably rhe best compromise between food detection ability (reduced metameric perceptions) and cost (in terms of neural resources subtracted to other task as well as reduction in other visula tasks like night vision itself). Here is an interesting link: http://www.handprint.com/HP/WCL/color1.html#quadchromat
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