This public-house sign represents the gamekeeper, who used at one time to be dressed in green.
But the `Green Man' shall I pass by unsung, Which mine own James upon his sign-post hung? His sign, his image— for he once was seen A squire's attendant, clad in keeper's green.
The men who let off fireworks were called Green-men in the reign of James I.
“Have you any squibs, any green-man in your shows?” —The Seven Champions of Christondom.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894