During the summer, leaves serve as site to produce food necessary for the growth of the tree. This food-making process occurs in numerous cells (found in the leaves) which contain chlorophyll. Chlorophyll is a chemical which gives the leaves its green colour, by absorbing energy from sunlight to transform carbon dioxide and water into sugar’s and starch. Along with the green pigments, yellow to red pigments are also present (carotenes and xanthophyll).
These pigments are what gives carrots their orange colour. In trees for most of the year, this pigment is suppressed by the green pigment. But in the fall, due to changes in daylight length and temperature, the leaves slow down their food- making process. This causes the chlorophyll to break down, resulting in the green colour disappearing and the yellow to red colors become visible.