As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter.
During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. As the bright green fades away, we begin to see yellow and orange colors.
The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves turn this glucose into a red color. The brown color of trees like oaks is made from wastes left in the leaves.
It is the combination of all these things that make the beautiful fall foliage colors we enjoy each year.
That was the scientific reason.
In the Bible, Solomon talks about the seasons changing which is found in Ecclesiastes.
I like to imagine God with his paint brush, painting the beauty of the trees in all his glory for us to see... He shows me that life is a masterpiece and we put the brush strokes on our journey thru life. Its up to us what colors we choose...and how we paint our life's story... discovering more and more colors and adventures. There's so much to see, so much to do! And I will never get tired of looking at them...