In autumn, the connection between the leaves and the rest of their plant begins to be blocked. This all happens at the time when the daylight hours become fewer, and when the dark nights become longer.
This is the time when a corky layer of cells forms near the juncture of the leaf and stem, slowly beginning to block the movement of minerals from the roots of a plant into the leaves. It is also the time for slowly blocking the materials such as carbohydrates from the leaves to the branches.
When the connection between leaf and branch is blocked, the production of chlorophyll slows and then stops - and the chlorophyll disappears completely.
This is the time when autumn colours are revealed.