Summer leaves are green because of the large amount of pigment chlorophyll contained in them. This pigment is the breadwinner of the plants, as it is through the light the plants from carbon dioxide and water synthesizes the main sugar - glucose, and all the other nutrients.
However, along with chlorophyll, the green leaves contain other pigments - yellow xanthophyll and orange carotene (the same one that determines the color of carrots). In the summer these pigments invisible, disguised as a large number of chlorophyll. In the fall the same as the attenuation of activity in leaf chlorophyll gradually destroyed. Here they appear in a sheet of yellow and red shades of xanthophyll and carotene.
The destruction of chlorophyll intensive is in the light, that is, in Sunny weather. That is why in cloudy and rainy autumn leaves longer retain their green color. But if the change protracted rains come "Indian summer", the crowns of the trees, for 1-2 day turn Golden colors of autumn.
Besides gold, autumn outfits trees contain crimson shades. This color makes the pigment called anthocyanin. Unlike chlorophyll, anthocyanin is not connected inside the cell with a plastic formations (grains), and dissolved in the cell SAP. With decreasing temperature, as well as in bright light amount of anthocyanin in the cell SAP increases. In addition, stop or delay the synthesis of nutrients in the foliage also stimulate the synthesis of anthocyanins.
Thus, the red color of the leaves during the fall, just shows what is happening attenuation of activity in the leaves, which are preparing for the fall...