The Gawain-Poet streses the colour green.  Why green?

The arms of the Order of Christ have a green background.  Afonso IV obtained papal permission for the Knights of Avis to bear a green cross.  João, the ultimate liege-lord of James Cottrell had been Master of the Order of Avis before taking the throne of Portugal in 1385.  Documents of the Exchequer calling in unpaid debts were sealed with green wax.  See Harley Lyrics III The Evils of taxation quoted in [TURPET89] (p.19).  Sir Gawain and the Green Knight is certainly about the payment of verbal contractual debts.