Fall is officially here, which means the leaves will change from green to shades of red, yellow, and orange. Buy why exactly does this annual event take place? Leaves change color as chlorophyll, the pigment that makes plants green, begins to break down in colder weather. That leaves behind other pigments, each of which produce a different color: Yellow leaves contain xanthophyll, orange leaves contain carotene, and red leaves contain anthocyanin. Fall foliage first appears in the northern U.S., and slowly makes its way south throughout October and November.