Red and blue clouds in deep space

RED

IC 1795, a star forming region in Cassiopeia
APOD ©2011 Bob and Janice Fera (Fera Photography)

"Ionized by ultraviolet light from energetic young stars, a hydrogen atom emits the characteristic H-alpha light as its single electron is recaptured and transitions to lower energy states." (APOD)

(Ionization is the process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or ions. An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. wikipedia)


H-alpha emission

Hydrogen alpha emission

In the simplified Rutherford Bohr model of the hydrogen atom, the Balmer lines result from an electron jump between the second energy level closest to the nucleus, and those levels more distant. The transition depicted here produces an H-alpha photon, and the first line of the Balmer series. For hydrogen (Z = 1) this transition results in a photon of wavelength 656 nm (red).
wikipedia

The visible hydrogen emission spectrum lines in the Balmer series. H-alpha is the red line at the right. wikimedia


Update: APOD 7 June 2012 has an impressive image of Venus transit taken with narrowband H-alpha filter. The red light from hydrogen emission shows interesting details on the surface of Sun not visible in regular photos.

BLUE 

Metrope's Reflection nebula in Pleiades
APOD ©2012 Leonardo Orazi

"The blue color typical of reflection nebula is caused by blue light being more efficiently scattered by the carbon dust than red light. The brightness of the nebula is determined by the size and density of the reflecting grains, and by the color and brightness of the neighboring star(s)." (APOD)