From MSNBC:
For years, scientists have studied how leaves prepare for the annual show of fall color. The molecules behind bright yellows and oranges are well understood, but brilliant reds remain a bit of a mystery. In response to chilly temperatures and fewer daylight hours, leaves stop producing their green-tinted chlorophyll, which allows them to capture sunlight and make energy. Because chlorophyll is sensitive to the cold, certain weather conditions like early frosts will turn off production more quickly.
Meanwhile, orange and yellow pigments called carotenoids—also found in orange carrots—shine through the leaves’ washed out green. “The yellow color has been there all summer, but you don’t see it until the green fades away,” said Paul Schaberg, U.S. Forest Service plant physiologist. “In trees likes aspens and beech, that’s the dominant color change.”
Scientists know less about the radiant red hues that pepper northern maple and ash forests in the fall. The red color comes from anthocyanins, which unlike carotenoids, are only produced in the fall. They also give color to strawberries, red apples, and plums. On a tree, these red pigments beneficially act as sunscreen, by blocking out harmful radiation and shading the leaf from excess light. They also serve as antifreeze, protecting cells from easily freezing. And they are beneficial as antioxidants.
More here.