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As the days get cooler and the amount of sunlight becomes less, trees produce less chlorophyll and the trees with carotenoid in their leaves begin to turn yellow and orange as the carotenoid begins to show through. Red leaves, though, are a different story.

During the day, sugar is still produced during the warm days of fall, but cool nights prevent the tree from getting the nutrients flowing from the leaves into the trunk of the tree. The Anthocyanins, which produce the red color, are produced to keep the leaves on the tree and the nutrients flowing until it's time for the leaves to fall off.

The color of yellow leaves remains the same from year to year, but different things such as temperature and the amount of rain, can make the red leaves brighter.

EEK (Environmental Education for Kids) has lots of information on leaves and why they change color.