Used to provide color in flowers. In autumn, deciduous trees stop synthesizing chlorophyll, and just let its degradation continue unmatched by new synthesis. So the green color isn't evident any longer.
Water stress can lead to early development of leaf color.
Chlorophyll disappears 2. Carotenoids, which were there before, then show up. This is the big story for aspen. But the leaves aren't the only story, and in fact, evergreens keep their leaves through the winter. Rapid freezing may be less harmful than slow freezing, because ice crystals stay smaller.