Emission nebula
A nebula's color depends on what chemicals are in it, and how ionized it is. Because most of the universe is made of hydrogen, many emission nebulae appear red. Hydrogen glows red when it is hot. If the nebula is very hot, other types of gas will be heated and the it might also glow blue and green. By looking at which colors are glowing in a nebula, one can figure out what it is made of.