Colours can quite often be someones interpretation of what they expect ( or would like ) to see. Longer wavelengths in the Infrared region can be discerned by filters but can be 'shifted' to a visble red in an image whereas visually they don't exist for the human eye. Similarly for Ultraviolet wavelengths, measurable by instrument only. Sure there are visible colours in nebula but they are also so faint that the eye cannot register them as such, hence your 'black and white' view.
I have seen faint colours in some of the major nebs, green and pink can be seen in M42 for example with a big aperture scope and good conditions. And during processing it is easy to shift colours to another wavelength to make them stand out from other characteristics of the image.
You can see this in peoples interpretations during processing. There are accepted conventions which we expect to see but they are not hard rules for sure.
I understand the direct photons viewpoint but it means the viewer may miss out on details that only show up with the long exposures required to see them.
Personally, I like both options. I get a thrill when I finally find/see/identify some obscure target dim and elusive but then I get another thrill when I capture it in detail and get a better unerstanding and more knowledge of what I have found.
It's all good fun..
