Leaf color begins to fade as the production of chlorophyll declines. Shortly after this happens, the remaining chlorophyll begins to decompose under the light of the sun. As the green disappears, other more stable pigments, such as carotene, are allowed to surface and the leaves begin to yellow. In some trees, as the concentration of sugar increases, it reacts with proteins in the cell sap to form anthocyanins. These pigments cause leaves to turn red, pink or purple depending on the pH level of the sap.
Robust oranges are a mix of anthocyanins, carotenes and xanthophylls.
Low temperatures eliminate chlorophyll and promote the formation of anthocyanins as long as they stay above freezing. The same is true of bright sunlight. That is why it is not unusual to see a tree with brilliant reds on one side and yellows on the other. Dry weather, which increases the sugar concentration in sap, also increases the amount of anthocyanins. Therefore, the most colorful autumn seasons are produced when dry, sunny days are followed by cool, dry nights.