Why leaves change its colour when they fall?

Changing Of Fall Foliage.

Leaves are plant’s food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into oxygen and glucose. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into oxygen and sugar is called photosynthesis. That means "putting together with light." A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green colour.

As summer ends and autumn comes, the days get shorter and shorter.  During winter, there is not enough light or water for photosynthesis. The trees will rest, and live with the food they stored during the summer. They begin to shut down their food-making factories.

Thus the absence of sunlight and cooler temperature makes the green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colours  Small amounts of these colours have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll.

Other chemical processes produce the brilliant reds, purples and bronzes. On warm fall days, sugar is produced in the leaves of some trees and then trapped by the chill of night. As sugar accumulates, the leaves turn brighter red.