Why do leaves change colors in the fall?

The simple answer is because plants stop producing chlorophyll toward the end of summer. The carotenoids, the pigments in the photosynthesizing cells, are present during the growing season but are overshadowed by the green chlorophyll while it is present. Starting in the fall, the trees don’t have enough water or light for photosynthesis so they start living off the food they stored during the warmer months (decreasing the chlorophyll).

As the green-colored chlorophyll goes away, we start seeing the other colors that have been there all along.

See Science Made Simple for a more detailed explanation.