One of the observable changes of a chemical change is the change in color. So, yes, leaves changing color is a chemical changes. This is brought about by biochemical reactions of the plant. The plant is composed of different pigments like chlorophyll, carotenoids and anthocyanins. The dominant one is chlorophyll which gives leaves its green characteristic cholor. The chlorophyll is abundant in plants because it is responsible for the process of photosynthesis. But when the season changes to longer nights and less sunlight, the chlorophyll cannot be used. So, it spends it reserves through other pigments. Hence, it changes color. A yellow leaf has dominant caretonoid pigment.
