Why do leaves change color in autumn?

Through spring and summer, leaves are packed with chlorophyll, the green pigment that captures sunlight for photosynthesis and masks other colours. As days shorten and temperatures drop in autumn, deciduous trees prepare to shed their leaves and stop making chlorophyll, which then breaks down. This unmasks yellow and orange carotenoid pigments that were present all along. In some trees, such as maples, sugars trapped in the leaves drive the production of red and purple pigments called anthocyanins. The brightest displays occur when warm sunny days are followed by cool but not freezing nights, which favours vivid red colours.