These pigments assist chlorophyll in absorbing sunlight and are present all summer but only become visible when chlorophyll is no longer dominant in the leaf. Anthocyanins are responsible for the scarlet, blue and purple hues in nature. They comprise the characteristic colours of Concord grapes, cranberries, blueberries, red apples, cherries, strawberries and plums but, unlike the carotenoids, must be produced within the leaf after the chlorophyll has declined. Anthocyanin are formed by the breakdown of excess sugars under bright light. Bright autumn days with cool nights favour the formation of anthocyanins, thus the connection between brilliant autumn hues and Indian Summer weather.
Tannins are the third prominent pigment.
They contribute the brown tones to the leaves, especially those of the oak and elm. After the carotenoids and anthocyanin have decomposed, the tannins alone remain, giving the dead leaves their characteristic brown colour. Certain colours characterize particular species. Leaves of some species such as the elms simply shrivel up and fall, exhibiting little colour other than drab brown. Autumn leaves of some trees such as birches, tulip poplars, eastern redbud and hickory, are always yellow, never red.