Sometimes in night photographies, we get some odd colors that are not natural but due to defects in the lens we use. These happen more with a wide aperture which is what we would use in night photos. A small trick is possible to fix this.

First, what is the problem? For instance this (badly focused) night photography closeup:

The aberration is visible on the left side of the white streak. The purple aspect is a clear indication of an optical problem and needs to be resolved. The other streak borders also show similar color problems.

The simple way of doing it is to filter the image, which will smooth the color but would not keep the crispiness of the image. So the way to do it is to then convert the smoothed result to HSL (Hue/Saturation/Luminance) space and then replace the luminance value by the original luminance value int he resulting image. This will result in the following correction:

The end result for the image would be something like:

The code can be found on GitHub as a set of ITK filters and can be compiled as a standalone application in AstroStack.