Greenish yellow in color used in bleach drinking water PVC pipes or in World War 1 as the world's firat war gas?

Chlorine. The Germans first used it. It moved over the battlefield like a green/white mist and it burned the eyes and lungs of those who got caught in it. Soldiers survived it by urinating into their handerchiefs and holding them over their noses, not pleasent but better than dying.