Solid image Marc, well done, well framed and great detail!
As to the above, (and with apologies for hijacking your thread) - if Ha is red ie: H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm - how it can possibly be green?
HST scientists simply assign Ha to the green channel, O3 to Blue and S2 to red simply for identification purposes, as it's easier to see the makeup of individual gases when assigned to these tones & blended than if they were all in monochrome.
To quote JP Metsavanio -
What is the truth then?
"Deep space targets are generally so dim, that human eye is not capable to see any colours from them. Closest to real colours is the RGB image, if colors are balanced correctly.
Usually a pure white Star is used to measure a correct colour balance between Red, Green and Blue data.
Next closest is the H-aRGB mix, where H-alpha filtered image is used to boost contrast and details in image. It tends to distort the Red balance in image though.
Third, in reality order, is a Narrowband mixture, where different channels are mapped to visible colours as closely as possible by the wavelengths.
Ha is mapped to Red with about 20% S-II, O-III is Green and the Blue is mixed from O-III and about 15% H-alpha to compensate missing H-beta.
The Hubble palette is least realistic colour scheme. It's made famous by wonderful pictures from the Hubble Telescope. This palette is developed to maximise visual difference between different ionised Elements in Nebula. Even though it's used in scientific purposes, it's beautiful as well. I use HST-palette for aesthetic reasons only, with that tool I'm able to reveal some of the hidden beauty.
Another good reason to use it is, that it's a de facto standard and makes possible to compare images from different narrowband imagers."
Now don't get me started on CFHT colour palette which is different again!
Point being, there are NO rules in Narrowband imaging colour choices. There are accepted "standards" eg: HST, CFHT etc. but that doesn't necessarily mean correct, aesthetically pleasing or scientifically accurate colours.
So then it's simply a matter of aesthetics and that which pleases the creators eye. If you like vivid, bold eye-catching palettes then go for it, or if you prefer blue and green without a colour in between and purple stars then that's your choice too!