The color green appears throughout the beginning of Sir Gawain and the Green Knight. The Green Knight was “all a glittering green” (26). His clothes and his horse’s clothes are green. In particular, “garments of green girt the fellow about – a two-third length tunic, tight at the waist…fitly held up hose, in hue the same green” (26 – 27). In the English tradition, the color green symbolizes nature, fertility, and rebirth (Wikipedia). In the medieval period, green also represented love and lust (Wikipedia). In combination with gold, green is a symbol of a youth’s passing (Wikipedia). Therefore, the Green Knight and his girdle can be viewed as symbolizing a youth’s passing. The color and size of the Green Knight may also refer to the Celtic Green Man who was the God of Fertility, a green giant (Morton; Love of Purple). As a result, the poem seems to be a confluence of Christian and pagan influences where the Green Knight represents nature and land and pagan influences while Gawain represents humanity trying to capture or control nature.
Love of Purple. http://loveofpurple.com/articles/l-gawain.html
Morton, Jill. A Guide to Color Symbolism. COLORCOM, 1997.
Morton, Jill. “The Evolution of the Symbolism of Green in Western Culture.” Color Matters. 2007. J. L. Morton. 06 Nov. 2007 <http://www.colormatters.com/culturematters.html>.