Same reason the colors of Chanukka are gold and blue....or halloween are black and orange, or Valentine's day red and white.....
Linda C

There's a link that discusses the possible answer, at http://www.seliot.com/archives/000037.html

But my personal guess is that, because many scholars feel that the pre-Christian links to the Christmas holiday strongly influenced what eventually became tradition, the red probably represents fire (and resulting light; both of which would have been a comfort in midwinter to peoples in whatever era) and the green represents evergreen trees which were a reminder and a promise of warmer days (and food crops!) at the other times of the year.