I am trying to reduce the impact of sodium vapour lighting on starscape-type images. This type of street lighting exhibits a very narrow orange spectral line, and it is common amongst astrophotographers to suppress that line using didymium filters. Now, I'd like to know how to get that effect by using digital post processing (aka "how to photoshop the ugly glare out").
An example image is below. The mill is lit by a single sodium streetlight behind my back, and the mist in the sky is lit by far-away streetlights. The wooden parts on top of the mill are painted white, so they can be used as a colour reference. Since there was no other light source around when the image was taken, removing the sodium spectral line should render the mill rather dark and those parts a relatively neutral grey.
For the sake of generality, and to be independent of any concrete software product, let me ask this way: How can a spectral line of a given wavelength be removed from a digital image in postprocessing?