The image below shows a comparison of conventional color rendering (left side) and the physically correct color rendering (right side). If you have ever observed M42 in a larger aperture instrument under good skies, you will remeber the grey-blue greenish tint of the outer regions of the Orion nebula, as you can see them in this image.

Monochromatic stimuli like Ha or OIII can not be adjusted by photometric color calibration methods, because those methods work based on white point calibration and therefore are only applicable for continuum emitters (i.e. desaturated colors) like stars, star clusters, galaxies or reflection nebula. Emission nebula, however, have to be numerically assigned the correct color, because they will be found on the very border of the color space and furhtermore are even outside the color gamut of dipslays and printing devices.

I developped this methodology over the past several years, beginning with a problem of colorimetric matching of different display technologies in one vehicle dashboard. Later, I extended this for narrowband emitters which can ,in a next step, perfectly be integrated into the astronomical imaging process. The last weeks being very cloudy gave me the time, to compile all my calculations and scripts into one paper.

To find out more and to learn about the necessary color transformations, you may read my paper on that topic, where I brought all the underlying mathematics together:

http://jupiter.n-t-l.de/knowledge_narrowband.htm

If you don't want to bother too much with theory, this does not matter: At the end of the paper you will find a table with the final results for all common narrowband emitters relevant in astrophotography! There you have the recipe, which color to assign to your narrowband channels. It is not up to one's taste, which color e.g. the Rosette nebula should get. It has its clearly defined color, which can be assigned to the Ha channel, in this special case.