Leaves are green because they contain chlorophyll. This substance helps plants use sunlight to make food. During spring and summer, trees make a lot of chlorophyll. Its strong green color hides the other colors inside each leaf.
In autumn, days become shorter and the weather gets colder. Trees prepare for winter and stop making chlorophyll. The green color slowly disappears. Then other colors that were already in the leaves become visible. Yellow and orange come from substances called carotenoids. These substances are also found in foods such as carrots.
Some trees make a red substance in autumn. Weather can change the final color. Sunny days and cool nights may produce bright red leaves, while cloudy or warm weather can make the colors less strong.