Leaves stop taking up water in autumn, preparing for winter dormancy. This lack of water to each leaf causes a very important chemical reaction to stop - photosynthesis - which is the food-producing combination of sunlight, water, and carbon dioxide. Chlorophyll depends on photosynthesis and disappears from the leaves. Chlorophyll is the green you see in the leaf.
Once the overwhelming chlorophyll color is removed, true leaf colors will dominate over the receding green pigment. True leaf pigments vary with the species of tree and thus the different characteristic leaf colors. And because true leaf colors are water soluble, that makes the color disappear very quickly after drying out.
Here is an illustrated step-by-step of how three major pigments color leaves in fall...