Colour display in green houses can be made with discharge tubes. The gas that is very frequently used for the display is

The gas that is very frequently used for colour display in greenhouses is neon.

Neon is a noble gas that is inert and non-reactive, making it an ideal gas for use in discharge tubes. When an electric current is passed through a tube containing neon gas, the neon atoms become excited and emit a characteristic orange-red glow. This glow can be used to create vibrant and colorful displays in greenhouses or other settings.

Other noble gases, such as argon, krypton, and xenon, can also be used in discharge tubes to produce different colors, but neon is the most commonly used gas for this purpose due to its bright and distinctive color.