Chris Peterson wrote:
Spectra tell you very little about color. The reason that the Owl Nebula looks quite blue in well exposed RGB images is because it isn't purely an emission nebula, but also a reflection nebula, with its dust reflecting and scattering the blue light from its very hot central star.
Infrared portrait of the Helix Nebula, highlighting dust.
Photo: Spitzer Space Telescope.
"Visual" portrait of the Helix Nebula.
NASA, WIYN, NOAO, ESA, Hubble Helix Nebula Team, M. Meixner (STScI),
& T. A. Rector (NRAO)
I doubt that reflection nebulosity affects the color of planetary nebulas.
Planetary nebulas are typically bluest near the hot central star. But there is usually not much dust near the central star, and not much reflection nebulosity there. The infrared, dust-highlighting Spitzer Space Telescope portrait of the Helix Nebula at left shows a "hole" in the inner part of the nebula. The picture at right, which shows a "typical visual" appearance of the nebula, is bluest where there is little dust.
In any case, even if there was
reflection nebulosity in the "blue" part of the nebula, reflection nebulosity is typically so faint that it doesn't stimulate the color-sensitive rods in our retinas at all.
Also bear in mind that there is probably a lot of dust in the coma of a comet. If dust makes the color bluer, the comet's coma should be bluer than it is.
One astrophotographer that I have the greatest respect for is Adam Block. Check out this page
to see a sample of his (old) planetary nebula images. Note that with one obvious exception, NGC 6826, all his pictures of planetary nebulas show them to be either greenish or reddish. I once wrote to him and asked him why his planetary nebulas look so green, and he explained that the color of OIII falls right between the green and the blue filters he was using at that time. On balance, and with the filters he was using, the planetaries were more green than blue.
(BTW, it also has a bluish cast visually when viewed in a large aperture telescope.)
I remember reading Burnham's Celestial Handbook, where he wrote that some people see planetary nebulas as green, and others see them as blue. Personally I have only ever looked at one planetary nebula through a telescope, and that was the Ring Nebula. It looked like a gray smoke ring to me.