Emission nebulae are essentially clouds of high temperature gas. Within the nebula, ultraviolet radiation from hot massive stars ionizes the gas in the surrounding cloud. Only very young stars produce enough ultraviolet radiation to ionize hydrogen, so emission nebulae are typically considered sites of active star formation. Eventually, the ions de-excite to lower energy states by recombining with the free electrons in the cloud. As the electrons fall to lower energy levels in the newly recombined atoms, they emit light at wavelengths corresponding to the transitions they make. The most prominent line seen in these nebulae is H-alpha resulting in their frequent red coloring. Another prominent line is the green ‘forbidden line‘ of double ionized oxygen [O III]. You can clearly see the red emission from hydrogen in the image below of IC 5067.