Leaves look green in summer because of chlorophyll, the pigment that drives photosynthesis. As autumn days shorten and temperatures fall, deciduous trees prepare for winter dormancy and stop making chlorophyll. When the green pigment breaks down, yellow and orange carotenoids that were already in the leaf become visible. In many species, newly made anthocyanins add reds and purples, especially after bright days and cool nights.
The same seasonal cues also form an abscission layer at the leaf base, sealing the twig and letting the leaf drop. So fall color is not a random paint job—it is the unmasking and, for reds, the brief production of pigments during a controlled shutdown for winter.
Source: Why do leaves change colors in the fall? — Encyclopaedia Britannica