When we think of traditional colors certain times of the year, those are actually the colors of nature at that time. In the wintertime, the trees you do see are evergreens. The only colors you generally see are bright red berries from an assortment of trees and shrubs. In the spring, the pale flowers that first come up are the colors we associate with Easter and spring clothing. In the fall, the fields and trees and squashes, including pumpkins, are the colors we think about with thanksgiving.

Edit- to add, many will point out the historical roots of Christmas trees, but those were still generally based on what was available in nature.