Depends on the filter used. I am assuming the author is using one of these hydrogens (probably Hα) and O-III and S-II. Each of these black-and-white images with

Depends on the filter used. I am assuming the author is using one of these hydrogens (probably Hα) and O-III and S-II. Each of these black-and-white images with this filter are used as the data for a single RGB channel in an image. Their wavelengths are, and would like like to the naked eye [1]: Hα (656 nm) bright red Hβ (486 nm) deep blue O-III (496 nm and 501 nm) greenish blue to green S-II (672 nm) deeper red The color mapping the author uses is called the Hubble Space Telescope pallet, where hydrogen = green, sulfur = red, and oxygen = blue. [1] http://pages.cs.wisc.edu/~yetkin/code/wavelength_to_rgb/wavelength.html http://pages.cs.wisc.edu/~yetkin/code/wavelength_to_rgb/wave...