You just can t see them in the summer because they are covered up by the green chlorophyll. The bright reds and purples visible in leaves are made mostly in the fall. In some trees, such as maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
Orange colors come from a substance called carotene and the yellows from xanthophyll.
They are common pigments, also found in flowers, and foods such as carrots, bananas and egg yolks. Although their exact role in leaves is uncertain, scientists think they may be involved somehow in photosynthesis. The bright red and purple colors come from anthocyanin pigments. These are also common in plants such as beets, red apples and purple grapes, and flowers such as violets and hyacinths. In the leaves, these pigments are formed in the autumn from trapped glucose.