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But, in the fall, the food factories shut down for the winter. The chlorophyll breaks down and the green fades away, letting the yellow/orange carotenoids blaze forth, giving autumn its splash, dash and panache.

The chlorophyll in the leaves starts to decrease. And remember, chlorophyll gives leaves their green color. So if there is less chlorophyll, the leaves lose their green color.

The chlorophyll breaks down with exposure to sunlight, so the leaf must always make new chlorophyll. As the Sun becomes less intense in fall, chlorophyll production slows down. This is why leaves change color in the fall. Plants need water, carbon dioxide, and sunlight to produce their food called glucose.

Chlorophyll Breaks Down But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor.

As the season changes, temperatures drop and days get shorter. Trees get less direct sunlight, and the chlorophyll in the leaves breaks down. The lack of chlorophyll reveals yellow and orange pigments that were already in the leaves but masked during the warmer months.

Less sunlight and cooler temperatures cause a biochemical reaction in the leaves. As the length of night increases, chlorophyll (green) production slows down and eventually stops. As this happens, the other two pigments (already present in the leaf) begin to show themselves.

The Short Answer: As summer fades into fall, the days start getting shorter and there is less sunlight. This is a signal for the leaf to prepare for winter and to stop making chlorophyll. Once this happens, the green color starts to fade and the reds, oranges, and yellows become visible.