The change in color of a leaf is primarily a physical change. This is because the color change occurs without any alteration of the chemical composition of the leaf.
During autumn, for example, chlorophyll (the pigment that makes leaves green) breaks down, allowing other pigments such as carotenoids (which can appear orange or yellow) and anthocyanins (which can appear red) to become more visible. The chemical compounds themselves may still be present in the leaf, but their prominence changes, which is a physical change in appearance rather than a change in the chemical structure of the molecules.