The gaseous nebulae, as I understand, have emission in mainly H-alpha
in red, H-beta in greenish-blue and OIII in somewhat greener blue. With
normal cone pigments, the peak sensitivities are in approx 420, 535 and
565 nm, the two latter in green (approx 2%, 1/3 and 2/3 respectively).
Thus, color perception must occur only after some processing - not
surprising to me if the threshold for perceiving e.g. OIII light as
colored could vary between individuals with "normal" color vision.

A simple experiment - illuminate a small test target such as a piece
of paper on black background with a green LED (constant current)
filtered by a OIII filter. Vary the distance and use the inverse square
law to determine the ratio threshold of color perception to absolute
perception limit with averted vision. Where do you see it at all -
where do you see it as colored - turquoise or what you call the color?
Does this ratio vary much between persons? I'd love to try for myself
but it will take weeks before I'm back to my "lab"...

But has anyone tried to see if any of the nebulosity in the bright
objects such as the great Orion nebula is visible at all through a H-
alpha filter? The parts I have perceived as reddish are the ones that
are least red on photos - it could well be smoe kind of "wishful
perception" of red...

Nils Olof