As the separation layer forms, the manufacture of food materials within the leaves slows down. The cells and veins in the leaf become clogged. No more chlorophyll is produced, and the green color disappears. Other colors were present in the leaf, but they were hidden by the stronger green. Now they appear in all their splendor.
All leaves contain yellow pigments called carotene and xanthophyll.
The yellow pigments are formed in the protoplasm of the leaf cells. The reds and purples are due to pigments called anthocyanins. They are formed in cell sap that is rich in sugar. Sugar maples, oaks, and sumacs have the most brilliant scarlet and purple colors. To develop such high color they must be exposed to the sunlight.