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Mark,
Colour is subjective and by no means of the imagination am I going to attempt to bring resolve to such a matter. In many cases it comes down to what and how the imager wishes to portray in an image. I know others will have thoughts on this topic.
There are minimal rules with narrowband imaging so will not cover these here, but getting a HaRGB blend right takes some work. Firstly, ensure you've got an accurate G2V weight ratio for your gear. If you don't, you're wasting your time to begin with.
Secondly, produce a straight RGB combine image using these G2V weightings. Gradients and other anomalies aside, a straight RGB is as close and as pure you'll get in terms of accurate colour. As soon as you add a luminance or Ha layer, the colours shift. Take a good long look at the colours of the straight RGB as this is ultimately what you want to replicate once you've added a Ha or luminance blend to it. The point of adding Ha data to the red channel is to simply colour the Ha data in luminosity layer. Nothing more, nothing less. How much you need to blend depends on how intense the Ha data is.
While it is ever so tempting to operate at 100% opacity with the Ha luminance for maximum detail, you'll find it hard balance/restore the colours back to what you saw with the straight RGB image. Try dropping the opacity percentage of the Ha luminance, flattening, boosting the saturation, then relayering the Ha luminance again at a higher percentage. There are a few other tricks to handling this.
The point here is what ever you see in your original straight RGB image should be your goal to portray in the final composite. Stretch it via DDP to get a feel for the dusty areas as opposed to the stronger Ha emission elements in the image. Anyway, this is the methodical process I utilise. I'm sure others will share their own.
Cheers
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