I understand the principle of how light is emitted from an atom. What I don't know is why neon atom is red and copper is green when exited? Is is the distance between the electron to protons or the number of electrons? When halogen bulbs get old the gas ionizes and emits a purplish hue compared to its normal white. Would a neon atom emit a different color if ionized?
The color of the photon is related to its frequency $f$, which can be related to the energy of the photon by the expression $E = hf$, where $h$ is Planck's constant. Thus the different colors of the emitted photons describes their different energies.
The next step is to determine why specific elements emit certain colors. This has to do with the different energy levels of the electrons "orbiting" the nucleus. When an electron drops from a high energy orbital to a low energy orbital the difference in energy results in an emitted photon. There are many rules regarding the allowed transitions, hence the allowed energy of photons that are emitted, and these vary by element. What you are observing are the allowed transitions of each specific element.