Introduction

Nebulae are clouds of gas in the universe. We find emission nebulae, rather often in the vicinity of hot stars. These nebulae emit light due to ionization of the atoms in the cloud or the high excitation of electrons caused by the unltraviolet radiation of young stars. The light of these nebulae is typically dominated by the red emitted by the HII transition (so from the third to the second lowest energy level).
Blue colors can be found in the so called reflection nebulae. Here we observer star light scattered by interstellar dust. As the dust scatters bue light more than red one, we actually observer a "bluer" nebula. We also observe dark nebulae: these are regions of dense gas clouds which hide the stars lying behind them in our line of sight.
In this section you will also see supernova remnants. These consist of the debris created by a star explosion, well, a supernova.

Images

NGC 6888	M16	M42	NGC 7000
NGC 6960	NGC 281	NGC 7023	IC 5070
IC 434 & NGC 2024 & B 33	IC 1396	IC1805	M78
M17	NGC 2264	NGC 2237/2244	IC1848

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References:
[1] J. Bennett et al., "Astronomy"