The yellowish lights are filled with sodium vapor. The blue-white lights are filled with mercury vapor. In both kinds, a high voltage spark jumps through the vapor, changing the gas into plasma, a different state of matter. Plasmas give off light. Common examples of plasma include neon signs, fluorescent lights, lightning and stars, including our Sun.

Different substances glow different colors when they are changed to plasma. Sodium plasma produces a bright, yellowish light.

With mercury vapor, we need another step. When changed to plasma, mercury vapor gives off ultraviolet light, which is also called "black light". Our eyes can't see ultraviolet light, but it can make some objects glow. Inside mercury vapor lamps, the glass is covered with a white powder that absorbs the ultraviolet light and glows with a blue-white light. The same is true for fluorescent bulbs, which is why they make such a mess when you break one.