Calling it "doctored" is perhaps a stretch. Hubble (and many earth-bound astronomers) take images of emission nebulae with narrowband filters covering some of t

Calling it "doctored" is perhaps a stretch. Hubble (and many earth-bound astronomers) take images of emission nebulae with narrowband filters covering some of the emission lines that are important and particularly strong. The typical ones are hydrogen-alpha, oxygen-III, and sulphur-II. These are particular colours of light that are emitted by these chemical elements, and so detecting these emissions are the best way to detect these chemical elements, for science. Then these detections are put together into a false-colour image so we can visualise it. We have three narrowband filters, so it makes obvious sense to map them to the three primary colours in the image. It's not about using weird filters on the telescope to make "doctored" images that look impressive. It's about using weird filters on the telescope to collect the best science possible, and then using that data to make an image.