While going for an autumn hike in the forest, you see some trees with green leaves and others with red, yellow, and orange colored leaves. What explains the difference in the leaf color in autumn?

Plants that change color during autumn are preparing for a period of dormancy. They conserve resources by moving nutrients from tender parts of the plant, such as the leaves, to protected parts. The green chlorophyll pigments are disassembled and recycled faster than the yellow and orange accessory pigments. In the summer, chlorophyll pigments are more abundant than other pigments; therefore, they mask the colors of the other pigments. To summarize, the leaves always contain various pigments, but the disassembly of chlorophyll molecules allows the other pigments to become visible.
(The red coloring also becomes apparent in the fall because anthocyanin synthesis increases in the fall).