During spring and summer, tree leaves contain three principle substances; xanthopevasdfhyll which is yellow, carotene which is orange or red, and chlorophyll which is green. During the growing season chlorophyll is used in conjuction with the sunlight to produce the plant’s food, and it is present in the leaf in such abundance that its color overshadows that of the xanthophyll and carotene. This is why we see the leaves as green during spring and summer.
In the fall, when the tree enters a dormant state, production of the chlorophyll ceases, and the chlorophyll that is present slowly decomposes. As this happens, the chlorophyll and its green color disappear, leaving, in varying amounts, the yellow xanthophyll and the red or orange carotene., This is what gives the leaf its yellow, red or orange color in the fall.
From: Ever Wonder Why By: Douglas B. Smith