Mustard Gas is a chemical weapon originally created by the Germans during World War 1 and 2. It is an incredibly dangerous gas in which it can kill pretty quickly, and 90% of victims die in around twenty to thirty seconds of direct exposure.
After Earth 1 was destroyed, a lot of it was discovered in a bunker in Germany, and it was distributed to the three strongest areas of Earth 2 - The USSR, the United States, and Great Britain, however it was made illegal unless there was a major conflict occuring.
It has only ever been used once on Earth 2, during a battle in Pakistan in 2006, in which Scottyvich Baloneykov used it on enemy soldiers and civilians illegally, bringing one of many charges on him that resulted in his removal from the Spetsnaz.
It's name comes from it's chemical name (Sulphur Mustard), and it's colour, which is generally a yellow-orange colour. It is an incredibly dangerous gas, and while not as deadly as ROFL-6, extreme caution is advised.