[Peter Ward]

I hesitated on chiming in here, but would suggest instead of just reading Clarke's web page, that you look at the work of Helmholtz and Maxwell who established colour theory about 175 years ago.

Malin's decidedly more contemporary "Colours of stars" is another excellent reference.

To suggest a "standard" off the shelf camera renders accurate nebula colour is nonsense. The white balance setting on a camera is there for a good reason. Ignoring the colour temperature of the source can easily bias the scene from red to blue, which if not compensated for, renders the scene a very "un-natural" fashion.

DSLR's are also fitted with cutoff filters that intentionally attenuate the IR plus red end of the spectrum and do not pass a significant portion of any h-alpha flux to their sensors, which would be swamped by even longer wavelengths. Balanced colour without this blocking filter is not possible. Owners of "full spectrum" modded cameras (self included) can attest to this.

So how does this relate to "natural" colour?

Normal human vision goes from photopic (daylight) mode driven by retinal cone cells (read colour sensitive) to scotopic (night time ) vision driven by rod cells which are most sensitive to wavelengths of around 498*nm (blue-green) and are insensitive to wavelengths longer than about 640*nm (red-orange).

In short humans don't perceive much colour viewing a landscape illuminated by a moonlit sky and as for scotopic viewing of faint h-alpha, well, forget it.

But, is the colour information still there? Of course it is. Our eyes can see 654nm wavelength red light ( e.g. observing the Sun through a h-alpha filter ) and deep violet, plus a myriad of hues in-between. Lack of illumination is not a good reason to assume what you perceive in the dark is accurate or "natural" colour.

The spectral flux emitted by nebulae while faint can be measured. RGB filtered images from the likes of the AAO used a matrix of ND filters to determine just how much flux was present in red blue and green exposures. This allowed RG&B exposures to be accurately calibrated and combined to deliver a faithful full colour image. Sure, there may be a slight calibration bias, but for example, the Tarantula Nebula emits mainly red light. Rendering it as blue-green is just plain wrong, yet this is exactly how most off the shelf cameras render it. By switching to an extended red spectrum camera, such as a Canon EOSRa, the nebula is recorded correctly as red. Interestingly the same camera when pointed at a daylight illuminated scene records "normal" colours with remarkable fidelity, albeit not quite as well as a standard model with daylight illumination.

If you really want to strive for accuracy, then a colour target chart can be used to calibrate your imaging system by varying RG&B exposures of the target, that when combined, best match the original chart. Once these exposure ratios are known, you can image deep sky objects with reasonable confidence that the results will indeed be accurate. Imaging software such as Pixinsight can also calibrate your colour data by using a star of known colour temperature to correct the image. Howver, care should be taken not to use field stars that are saturated (i.e. burnt out) with this method.

All that said, narrow band data can also be used to either enhance RGB images or be combined to produce SII, Ha and OIII imagery. But while adding H-alpha (i.e. red) to green or blue channels might produce an interesting result, it ignores the physical reality. A bad move IMHO. Just because you can, doesn't mean you should.

With narrow band imaging it's dealer's choice as there is no "natural" colour filter combination that can give you a full-spectrum image. However I'd suggest using an eye-pleasing palette rather than a combination that results in muddy brownish result.