Fall colors in trees and shrubs explained:
Fall color is controlled by genetic factors and the environment. Carotene and xanthophyll are yellow pigments produced all year long as well as the green pigment chlorophyll. As days get shorter and cooler, less chlorophyll is made and it breaks down and disappears to reveal the other pigments that were there all summer. Some plants produce red and purple pigments knows as anthocyanins and this pigment increases as sugar increases. Sunny days and cool nights favor sugar production and this intensifies the red color. Tannins in oaks account for tans and browns which also appear as chlorophyll disappears. Frost and below-freezing temperatures will stop, not start, color formation and will kill leaves.