As temperatures begin to cool and the days get shorter, leaves on deciduous trees begin to change colors. But why? There are several hypotheses for this phenomenon, but it is likely a chemical reaction that is triggered by the amount of daylight the plant is receiving each day. During the summer, leaves are busy producing sugars and starches for the plant to consume using a cell called chlorophyllwhich gives the leaves their green color. Inside the plant, chlorophyll cells use sunlight as energy to convert carbon dioxide and water from the air into carbohydrates through a process called photosynthesis. Because there is less daylight when autumn starts, the process slows down, and the chlorophyll cells begin to break down.  

The wide variety of reds, yellows, and oranges we see slowly appearing on leaves during this time of year is not produced from the leaves decaying; these pigments are already a part of the leaf! We cannot see these colorful pigments during the spring and summer months because they are covered by the green chlorophyll pigment. Once the leaves reach a point where they are not absorbing enough sunlight to continue photosynthesis, the chlorophyll begins to break down and reveal the other pigments making up the leaf. When the leaves fall off, the plant remains in a state of dormancy until the days begin to get longer again. Once spring comes around and the plants start receiving longer periods of daylight, buds will begin to burst, and this process will begin all over again!  

Check out the article here:
Why Leaves Change Color