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Bottom line: we can see rudimentary colors in mesopic vision (bright planetaries and maybe the trapezium). I doubt we can see the red at all (L-cones are far little sensitive to H-alpha).

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A few planetary nebulae do show some red color in moderate to large apertures. The most famous example is probably IC 418 in Lepus, which is sometimes known as either the "Pink Planetary", or the "Raspberry Nebula". The outer shell is often distinctly reddish in hue at low to moderate powers, while the central core is more bluish in color. Another example is the tiny planetary Campbell's Hydrogen Star (PK64+5.1) in Cygnus, which, at least in my 9.25 inch SCT, looks about as reddish as an M-class star (looks a little like the companion to Eta Cassiopeiae). As for diffuse nebulae, the parts of M42 which show reddish colors to me tend to be well *away* from central Huygenian region and the Trapezium rather than adjacent to them. These reddish regions tend to be narrow and often located the outer regions of the nebula in places similar to those shown in photographic images. It does take some aperture to see the coloration, but it does show up from time to time depending on how sensitive the observer's eye is to dim red light. Clear skies to you.

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David W. Knisely
Hyde Memorial Observatory
http://www.hydeobservatory.info

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