Why does a sodium street lamp give off a yellow color whereas a neon light gives off a red color?

The color of the light depends on the element to which it interacts. On interacting with energy, the electrons goes to the higher excitation state and when electron drops to lower state which is more stable, the energy is re-emitted as light. Each element produces its own characteristic color.

In case of a sodium street lamp, more photons in the long wavelength are emitted by the sodium gas, leading to yellow color one see.  Whereas in case of a neon light, the neon gas is a noble gas, stable and non-reactive. The electrons in neon will excite on applying current. The free electrons will collide with each other and then return back to the atoms. The energy is produced in the form of light on absorbing electrons by the atoms. This falls in the red region of spectrum and thus emits red color.

explanation:

metals have excess of electrons whereas non-metals have deficiency of electrons. thus, in order to gain stability metals easily donate their electrons to the non-metals.

for example, electronic configuration of sodium is and electronic configuration of phosphorous is .

when a phosphorous atom and sodium atom combine together then sodium will donate is extra one electron to the phosphorous atom. as a result there will be formation of ionic bond between sodium and phosphorous.

the reaction will be as follows.

there, are three sodium sodium atoms and each of them will donate its one extra electron to phosphorous atom. thus, the sub-shell of phosphorous gets completely filled.  

thus, we can conclude that electrons are transferred from atoms of sodium to atoms of phosphorous. this transfer makes the sodium atoms donate one electron and the phosphorous atoms to gain one electron. as a result, the sodium and phosphorus atoms strongly attract each other.