Many years ago I postulated that the reason for the vibrancy of the fall leaf colors is not due to moisture content but rather to ambient temperature. That is, cold nights retard the movement of carbohydrates from leaf to root.
The above is true but If remember correctly a sugar reaction happens in maple leaves and they change color.
Leaves do not change color in the Fall.
OK, they do change their overall colors.
Leaves of deciduous trees appear yellow, red, an eventually brown when they die.
The reason that leaves’ colors shift is that when the weather gets cold enough, they leaves stop functioning as food producers for the tree. The tree becomes dormant preparing to survive the cold extremes of Winter.
During the Spring leaves develop and grow into food producers for the entire tree. In the Summer, leaves are in full production.
The green color you observe in Spring and Summer is a composite of all colors of all the chemicals that make up leaves. Chlorophyll is the predominate chemical in most deciduous trees. Chlorophyll is green. It is chlorophyll that uses sunlight to manufacture nourishment that feed the entire tree, including leaves, branches, trunk, and root system.
In the Fall, as temperatures drop, leaves lose their ability to produce nourishment when their cellular structures have been damaged by frost conditions. Once damaged, the leaves lose their chlorophyll — and therefore their green.
As the chlorophyll is no longer produced, its green color is no longer present. That allows other colors to take over the total color appearance of the tree’s leaves.
Some of the remaining chemicals’ colors are red and yellow. The combination of these chemicals will change during Fall, thereby making the leaves appear to shift in color over a few weeks in Fall.
The color remaining after other colors diminish makes leaves look different than before the cold caused photosynthesis stop.