Emission nebula have red color in common it is because they all share abundance of hydrogen.
Emission nebula are the clouds of ionized gas which emit their own light at optical wavelengths. Some types of emission nebula are H II region, planetary nebula, reflection nebula, dark nebula, etc. Dark areas in nebula are the clouds of dust which block the light.
Color of emission nebula depends on its chemical composition and degree of ionization. Blue and green colors can be produced by the atoms of other elements but hydrogen is always the most abundant.
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