The answer
As days shorten, trees stop making green chlorophyll, revealing yellow and orange pigments that were there all along, while some trees also make new red ones.
Leaves are green because they're packed with chlorophyll, the pigment that captures sunlight to make food for the tree. Chlorophyll keeps breaking down in the sunshine, so all summer the tree keeps making more.
Leaves also hold yellow and orange pigments called carotenoids, the same family that colours carrots. In summer there's so much chlorophyll that you can't see them.
As nights get longer and cooler in fall, broadleaf trees get ready for winter. They stop making chlorophyll, and the green fades. The yellows and oranges that were hiding underneath show through.
Some trees, like red maples and sumac, also make new red and purple pigments called anthocyanins in fall, from sugars trapped in the leaf. Sunny days and cool (but not freezing) nights make more of them, which is why some autumns are especially red.
Go deeper
Why? Why do trees drop their leaves at all?
In winter, water in the ground can freeze and sunlight is weak, so big thin leaves would lose more water than they're worth. Trees seal off each leaf with a layer of cells at its base, and the leaf falls.
Why? Why don't pine trees change colour?
Most conifers have needles with a thick waxy coat that loses little water and survives freezing, so they keep them for several years and stay green.
Why? What makes the brown colour?
Brown comes from tannins, waste materials left in the leaf after the other pigments are gone. Oaks are known for their brown leaves.
Keep wondering
Sources
- Why Leaves Change Color, SUNY College of Environmental Science and Forestry
- Why Leaves Change Color, USDA Forest Service