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Emission nebulae are usually red, because hydrogen, the most common gas in the universe, most commonly emits red light. Reflection nebulae are clouds of dust that simply reflect the light of a nearby star or stars. Reflection nebulae are usually blue, because blue light scatters more easily.Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae.In some nebulae, the star formation regions are so dense and thick that light cannot get through.eflection nebulae are usually blue, because blue light scatters more easily. Emission and reflection nebulae are often seen together and are sometimes both referred to as diffuse nebulae. In some nebulae, the star formation regions are so dense and thick that light cannot get through.
It's a simple question of scattering. The reflection nebula gain reflectivity due to particles of Carbon (usually diamond dust), Iron and Nickel. Now the size of these particles is very small, and is more comparable to the wavelength of Blue light rather than Red. Hence the blue end of the spectrum is scattered better while the Red is absorbed better. So they appear more blue than red. The frequency distribution in the light of these nebulae is similar to that of the illuminating star(s), which provides significant evidence that it is not self-illuminated, and is reflecting light of that particular star.