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 
I think you meant Red Green Blue not Red Green Red.
Hi D.Rice, thank you for your comment. Both Hydrogen and Sulfur have as real color the color red. Because this is problematic, for the Hubble palette, we apply ‘false’ color mapping, see also text below the table.
Thank you Bart, this is so interesting. It would be clearer from the start if a “false color” column was added to the chart.