Why Leaves Change Color
Most leaves have a colorless epidermis; their green color is due to the chlorophyll, which contains a green pigment, within the mesophyll. In plants such as the coleus, however, the cells of the epidermis contain other pigments. The leaves are the colors of those pigments rather than green.
Leaves that are normally green turn yellow, brown, orange, gold, red, blue, or purple when they die. This change is caused by the disappearance of chlorophyll from the leaves. Decreasing sunlight reduces the amount of chlorophyll and slows photosynthesis in the leaf. The receding chlorophyll reveals pigments that were already in the mesophyll but were masked by larger amounts of chlorophyll. The yellow, brown, orange, and gold colors are produced by three groups of pigments: xanthophylls, (yellow), tannins (brown), and carotenes (yellowish orange to light red).
Blues, purples, and deep reds are produced by pigments called anthocyanins. Anthocyanins are produced by the action of sunlight on sugar and other substances within the leaves. They are produced in great quantities in the sugar-rich leaves of sugar maples. These are among the most brilliantly colored of all autumn leaves. Much sunlight is needed during autumn to produce bright colors. An early frost is also necessary to produce bright colors since it prevents the sugary fluid in the leaves from draining into the stems.