Here's why:

* Hydrogen: Emission nebulae are primarily composed of hydrogen gas.

* Excitation: The hydrogen atoms in these nebulae are energized by nearby hot stars, causing them to become excited.

* Photon Emission: When the excited hydrogen atoms return to their ground state, they release photons of light. The most common emission is a red photon corresponding to the Balmer alpha line.

While the red color is most prominent, other elements in the nebula can contribute to other colors:

* Oxygen: Can contribute a blue-green hue.

* Nitrogen: Can add a reddish-pink color.

Therefore, while the primary color is red, the overall appearance of an emission nebula can vary depending on the composition and the intensity of different emission lines.