Fall color is caused by a combination of leaf pigments, length of day, types of tree, genetic variation, and weather. All tree and plant leaves contain pigments that give them their color. During the growing season, most tree leaves are green because they are full of chlorophyll that the tree uses to capture sunlight for photosynthesis. These high levels of chlorophyll mask other pigments present in the leaves. As the days grow shorter in the fall, chlorophyll production slows down and stops. The other pigments present in the leaves then become visible.

Different species of trees typically turn specific colors. For example, aspen turn yellow, and sugar maple turn orange-red. Also, timing of color change is species-related, and seems to be genetically inherited. For example, oaks turn color after other species have dropped their leaves.

Temperature and moisture have a great influence on autumn color. Consecutive days of warm, sunny weather and cool nights bring about the most spectacular fall colors. A late spring or drought can postpone fall colors. After deciduous trees turn color in the fall, they drop their leaves in order to survive harsh winter conditions.

more info: http://www.na.fs.fed.us/spfo/pubs/misc/autumn/autumn_colors.htm