Why do leaves change color?

It's that time of year again when the leaves change from green to beautiful shades of red, orange and yellow. After the autumnal equinox, deciduous trees (trees that seasonally shed their leaves) begin to expose their many hidden pigments. Simply put, the plant stops its food making process (photosynthesis) that takes place during the spring and summer. The change in temperature and length of daylight signals the plant to become less metabolically active, no longer needing the green colored pigment chlorophyll that is necessary for plants to create oxygen and glucose.

As chlorophyll declines, other pigments in the plant material become more abundant. A pigment is a substance that appears a certain color to us depending on its molecular composition. This composition controls which wavelengths of light are absorbed and emitted (the color we see) from the leaf. The fall colors we see are biological pigments, or biochromes that include xanthophylls (yellows), carotenoids (oranges), and anthocyanins (reds). 
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Next time you peep the colorful leaves, don't forget to ask yourself which pigments are controlling the wavelengths of lights hitting your cornea. Happy Fall!