"neon"

[dg_5021]

What is the immediate source of the colored light produced by "neon" signs?

[Sirus]

What exactly do you mean by "immediate" source? Neon signs work by running a stream of electrons through a tube filled with a gas. The electrons in the gas particles are excited (bumped up to higher energy levels) by the electrons in the stream, and as they fall they emit the energy in the form of light. The colour depends on the frequency, which depend son the amount of energy possessed by each photon of light, all of which depends on the specific energy levels of the gas particles (so the type of gas) and the voltage across the tube.

[chem_tr]

I think fluorescence occurs in a vacuum tube with two electrodes in the presence of neon gas (this is a brief description of neon lamp). Sirus' explanations are sufficient in theory, if you don't look for more.