A. The glass tube is filled with a gas at a low pressure. Electrodes at the ends of the tube are connected to a source of high voltage AC, typically 30 kV for a small sign. The high voltage ionizes the gas. Electrons are pulled out of the outer orbits of the gas atoms, resulting if free electrons and positively charged gas atoms.
Both of these serve as charge carriers, and support the flow of current through the tube. When electrons fall back into orbit around the ionized gas atoms, photons of light are emitted.
The wavelength of the photon, and hence the color of the light, is determined by the amount of energy the electrons give up when they fall into orbit around the gas atoms. This amount is different for different kinds of gas atoms. Thus, different gases produce different colors of light. Neon give a reddish-orange color, while Krypton gives off a green color, and Argon white. Different blends of gases are also used to produce different shades.
Small neon indicator lamps for panels were popular before the advent of LEDs. These were usually red, although yellow and green were available. About 70 to 80 volts was required to make them conduct, and the voltage across them stayed about the same for a wide range of currents. Usually, they were operated off the AC power line. A current limiting resistor was placed in series with them. They are still used on some types of industrial equipment and vending machines.