Usually the recombination occurs at one of the upper energy levels of hydrogen and a low energy photon low energy photon is released. Very quickly, however, the electron cascades down the energy levels toward the ground state, releasing photons of higher and higher energy. When the electron falls from the 3rd energy level to the 2nd energy level it emits an H-alpha photon, which is very red. This very common transition gives diffuse nebulae their red glow. Many other transitions, including those of other elements can also be seen at different wavelengths, but hydrogen is the most common element, and H-alpha is the most common transition visible in the optical, and this is why in most photographs, HII regions appear red.
Ionized gas is also common in supernova remnants.
Supernovae are the dramatic and explosive ends to the lives of massive stars. The explosions are so violent that they literally blow bubbles in the interstellar medium. These last remnants of the stellar envelope plow outward at hundreds of kilometers per second into the surrounding interstellar medium, forming a supersonic shock. The shocking heats and ionizes the gas, which, in turn, radiates in the optical, as seen in this photograph of the Vela Supernova Remnant . The red color derives once again from the H-alpha transition.