hydrogen
If the person posting the question is looking for a gas that is an element perhaps the answer they are looking for is Chlorine [Not that Hydrogen is not an element but I do not recall it having colour]
Chlorine gas is yellowish green. It's a yellowish white when a solid.
Sulfur is a yellow element that smells of rotten eggs.
it is a yellow green gas
Sulfur
Sulfur
Sulfur
Chlorine gas Cl2
Chlorine in its elemental form is a greenish-yellow gas. However, when it is dissolved in water as chlorine gas, it gives the water a yellow-green color. In a solution such as bleach, chlorine appears as a pale yellow or yellow-green color.
yellow-green
The gas commonly used to make yellow-green neon signs is neon gas. When an electrical current passes through neon gas inside a sealed glass tube, it emits a bright yellow-green light.
I suppose that the gas is argon.
The question is actusally related to the mystery of "What color is an orange ?"