In neon signs, argon gas is used for the Blue color.
Neon gas is associated with neon signs and is actually used to produce the orange-red color in these signs.
To create a blue color in a neon sign, a small amount of mercury vapor is added to the argon gas inside the glass tube.
When an electric current is passed through the gas mixture, the mercury vapor emits ultraviolet light, which causes the phosphor coating on the inside of the glass tube to fluoresce and produce a blue color.
It's important to note that while argon gas is commonly used to create the blue color in neon signs, other gases such as helium, krypton, and xenon can also be used to produce different colors in these signs.