One suggestion is that the gas in the filaments is colliding at high speed with other interstellar gas that is cold and thus not visible on the prints. The collisions then ionize the atoms. The region of actual collision is usually sharply defined and has higher density and is therefore brighter than surrounding regions. Another suggestion is that the gas was ionized by a burst of radiation from a supernova. The gas has not had time to cool down and is still shining, though the supernova faded long ago.
Dust reddens star light passing through it by scattering the blue light in all directions.
If the dust is close to a star then we will see the blue light reflected as a blue nebulous region surrounding the star. Occasionally gas is thrown off by stars forming a slowly expanding shell of gas surrounding a very hot blue star. The H emission line is strong in red, and the oxygen emission lines will produce a very faint blue image.