Only if surface brightness is above L-cones threshold (say 17 mpsas) you have a chance to see real colors. Otherwise there are solid scientific studies that show that color perceptions at scotopic levels (say 20) happens but are illusions. But, even if the surface brightness in V band is above the typical figure of 17-18, that does not necessarily mean that the red fraction (H-alpha emission) is strong enough to stimulate L.cones. L-cones are weakly sensitive to H-alpha. Thus it might be that reddish percepition on nebulae that are on "average" above the theoretical limit for cones is still an illusiuon.
A proof might be to look at the nebula with a H-alpha filter. If you cannot see it then your L-cones are not stimulated by the H-aplha emission (rods do not see H.alha at all). You only detect OIII and other lines with rods.
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Re: Is it possible to see the nebula with colour?
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