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.

a corroding iron, if that's one of your choices

the answer is symmetrical : )