Emission nebulae are some of the brightest objects in the night sky, the Orion Nebula shown above is visible to the unaided eye and in good skies even some of its colour shines through.
The light from emission nebulae is special in that it is produced by the energetic nebula itself, rather than being reflected or blocked light from nearby stars, and the Orion Nebula is no different.
When atoms of a gas are excited, in this case by the strong radiation of young stars, the electrons transition to higher energy levels. This excited state however is often not stable and will quickly decay, the electron dropping back down to a lower level. Energy must be conserved so as the electron loses its energy it must release a photon of light to carry away this energy. Each transition has its own associated energy and every atom has its own transitions so the energy of these emitted photons is unique to that particular atom, the emission spectra of the atom. In astronomy these correspond to now well known emission lines such as Hα SII and OIII. Not every transition is allowed as certain quantum properties prefer to be conserved.
The energy of a photon directly relates to its colour, lower energy being closer to red and higher more towards blue. The red light seen in the centre of the Orion Nebula is Hα, with a wavelength of ~656 nm, a emission line unique to a certain electron transition in hydrogen. The blue colour seen in the surrounding gas comes not from emission but reflection of the light from the young hot stars being born in the centre of the nebula.
However it was noticed in the early 20th century that there was another colour in the nebula, a green-cyan colour, which could not be explained by any known emission lines. By this point the emission of almost every known element was very well documented so some theorised there was a new element present in the nebulae, dubbed Nebulinium. No such element was discovered.
It wasn't until later in the century that Ira Sprague Bowen discovered the cause of this light was not due to an unknown element but a relatively mundane gas which is present in our own atmosphere. The emission line was due to oxygen. Specifically a transition in a doubly ionised oxygen atom (OIII) and the reason that this transition had not been observed before is that it is one of the forbidden transitions mentioned above, the reason these transitions are unlikely is that the state of the element during the transition is meta-stable and it lasts for a few seconds rather than the nanoseconds of other transitions. On Earth in one second the element will almost always interact with another particle and transfer away its energy without emitting a photon but in nebulae like the Orion nebula the gas is so spread out and diffuse that these states survive long enough to release their light.
Only in these pristine conditions can the classically forbidden light of the Orion nebula be released.