Does anyone know why the colors of Christmas are usually red and green??
Same reason the colors of Chanukka are gold and blue....or halloween are black and orange, or Valentine's day red and white.....
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.