When the gas attack suddenly descends on the troop of soldiers, who are described in terms that make them represent anything but soldiers, chaos descends on the company as they struggle to put on their gas masks and protect themselves from the immense danger of the gas. The description of the speaker of the poem of what he sees through his gas mask when one of his company fails to place his mask securely on his face in time is a chilling description of man dying:
Dim, though the misty panes and thick green light,
As under a green sea, I saw him drowning.
Thus we can see from this description that the gas is green in colour. The comparison the poet makes is to compare the man dying with a man who is drowning. The green gas in the speaker's mind suggests green sea water, and being gassed, like drowning, is a death that takes place gradually as oxygen disappears.