I was just about to comment more on the title that is a bit misleading. This has nothing to do with relativity.
Moreover, after thinking about your article, I am not sure to agree. The atom absorption spectrum equals the atom emission spectrum. Then, a given atom species will absorb light from a particular wavelength and reemit isotropically. This is the color the atoms are. If I am not wrong.
A difference can also occur at the level of molecules, where the emission spectrum could be different from the absorption spectrum.
@justtryme90: do you mind commenting and helping? :)
EDIT: forget what I said (see here). Today I learned something! :D