Answer: The seasonal color change has to do with chemical changes inside the foliage. Those that are sensitive to the shortening day length and to cooler temperatures undergo seasonal change as preparation for their winter survival; as chlorophyll levels drop the remaining pigments in the leaf become visible.
Here is a more detailed explanation of what happens. You may need to cut and paste the complete url into your browser to make it work correctly.
For a slightly less technical explanation, try this one.
I hope this answers your question.
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