Why are some leaves red, and others yellow or orange?

Other chemical changes may occur and cause the formation of additional pigments that vary from yellow to red to blue. The colors are due to the mixing of varying amounts of the chlorophyll and other pigments in the leaf during the fall season.
Fall weather conditions favoring formation of brilliant red autumn color are warm sunny days followed by cool, nights with temperatures below 45o F. Much sugar is made in the leaves during the daytime, but cool nights prevent movement of, sugar from the leaves. From the sugars trapped in the leaves the red pigment called anthocyanin is formed.

The degree of color may vary from tree to tree. For example, leaves directly exposed to the sun may turn red, while those on the shady side of the same tree or on other trees in the shade may be yellow. The foliage of some tree species just turns dull brown from death and decay and never shows bright colors.

Also, the colors on the same tree may vary from year to year, depending upon the combination of weather conditions. The most vivid colors appear after a warm dry summer and early autumn rains which prevent early leaf fall. Long periods of wet weather in late fall produces a rather drab coloration. Droughts favor anthocyanin formation principally due to the indirect effects of soil water deficiency upon the metabolism of the plants. Drought conditions also favor red pigment formation due to the reduction of nitrate absorption.

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