Leaves are colored by molecules called pigments. Chlorophyll pigment is the one makes the leaves green and is important for plants to get food utilizing sunlight. During the spring and summer when there is the excess of sunlight, plants produce a lot of chlorophyll. But during the winter season when it gets cold, dry, and normally there isn't much sunlight, some plants cease making chlorophyll. Rather, those plants break down chlorophyll into smaller particles. As the amount of chlorophyll gets lower because of lesser sunlight, other pigments begin to show their colors. That is why leaves get yellow or red in winters or fall.