What element is used in advertising signs?

It sounds like you are referring to "neon" signs. About 1/3 of all "neon" signs actually use tubes filled with very low pressure neon. The rest use other noble gases like helium, argon, krypton, and xenon - often with something like a minute amount of Mercury added and a phosphor coating to the glass so that the resulting UV light produced when the gas is electrified can be converted to other colors by the phosphor coating (which fluoresces when bombarded by the UV).