According to the passage, why do yellow and orange colors appear in the fall?

In much of Texas, fall arrives quietly. But in a few places, such as Lost Maples State Natural Area in the Hill Country, hillsides of maple trees turn red, orange, and gold each autumn, and visitors drive for hours to see them. Why do leaves change color at all?

The answer begins with green. Leaves are green because they are full of chlorophyll, a substance that captures energy from sunlight. Plants use that energy to make food from water and carbon dioxide, a process called photosynthesis. During spring and summer, a tree keeps making new chlorophyll, so its leaves stay green.

Green is not the only color in a leaf, though. Many leaves also contain yellow and orange substances called carotenoids, the same kind of substance that makes carrots orange. All summer, the strong green of the chlorophyll hides them.

As fall approaches, the nights grow longer, and in many trees cooler temperatures add to the signal. The tree stops making new chlorophyll and begins to prepare for winter. As the green fades, the yellows and oranges that were there all along finally show through.

Reds and purples work differently. In some trees, including many maples, the leaves make new substances in the fall, called anthocyanins. Sunny days and cool nights that stay above freezing tend to make the reds brighter. That is one reason the colors can change from year to year, even on the same tree.

Eventually, the tree seals off each leaf where its stem meets the branch, and the leaf falls. The tree rests through the winter and grows new green leaves in the spring.