In a very topical episode of Minute Earth, narrator Henry Reich, explains with the help a vivid animation ,the real reason the leaves of deciduous trees change color in autumn before falling completely off their branches in winter. It starts when the cells of the leafs halts the photosynthesis process.
As the disassembly starts, the chlorophyll molecules that absorb sunlight for photosynthesis are still absorbing the sun’s energy. But with no photosynthesis happening, they end up passing the unused energy along to oxygen molecules, which become dangerously reactive when energized. These molecules wreak havoc, damaging the parts of the leaf that are in the process of recovering and transporting nutrients back to the rest of the tree.To keep this destruction to a minimum, leaves break down their chlorophyll into less dangerous molecules that are typically transparent, though sometimes yellow. Either way, with the bright green molecules gone, yellow and orange pigments that were there all along (helping with photosynthesis), are no longer overshadowed, and ta-da! Yellow and orange leaves. Some trees take an extra autumnal precaution against chlorophyll-induced-destruction: as the leaf dismantling starts, they build new, special pigments to shade chlorophyll from sunlight until it can be broken down. These new pigments tend to be red or purple in color, so trees that use them have red leaves in the fall – sometimes insanely bright red.