False-colour images of nebulae are essentially RGB images whose colour channels have been mapped to specific emission lines. In these images, each colour can represent a specific element. In other words, a false-colour image of a nebula tells us exactly what it’s made of. There are many emission lines, but the three most commonly photographed by astronomers are hydrogen-alpha, oxygen-III and sulfur-II. These emission lines are captured by using narrowband filters which only let through the light at very specific wavelengths, typically with a bandwidth of 12µm or less.
| Element | Emission line | Wavelength | Colour |
|---|---|---|---|
| Hydrogen | Hα | 656.3 nm | Red |
| Oxygen | O-III | 500.7 nm | Green |
| Sulfur | S-II | 672.4 nm | Red |
Mapping Hα, O-III and S-II to red, green and blue is problematic when two of them are red, one is green and none is blue. Astronomers deal with this by using false colour — one or more of these elements is going to have to take a hit for the team and take on an unnatural hue. The Hubble palette assigns red to S-II, green to Hα, and blue to O-III: red is accurate, green and blue are false.
Bart Delsaert - Astrophotography 