An emission nebula is a cloud of gas and dust that gives off its own light after a nearby star or stars emits enough ultraviolet radiation to strip the atoms in the cloud of some of their electrons. Emission nebulas tend to be rich in hydrogen, oxygen, and sulfur gasses. Each of those elements produces light in very specific, very narrow wavelengths, which allows astronomers to use specialized filters to capture that light. 

Ionized sulfor emits a deep red light near the infrared portion of the spectrum. Ionized hydrogen gives off a brighter red light, closer to the green part of the spectrum. And ionized oxygen gives off a teal-colored light that straddles our perceptual line between green and blue. In the early 1990s, the Hubble Space Telescope team devised a color palette—unsurprisingly dubbed the "Hubble palette"—that assigns these emissions to color in the RGB color space in spectrum order. Hence, light from sulfur is assigned to red, light from hydrogen to green, and oxygen light to blue. 

This produces a "false color" image—but what it expresses is far from false. This assignment of color allows the astrophotographer to expose the true structure and chemistry of the nebula in a way the human eye and brain can easily apprehend. The combination of sulfur and hydrogen in this palette often produces brilliant hues of gold or deep shades of brown while the combination of oxygen with the other two elements can create pink, magenta, teal, and even purple coloring.