Final answer:
Carotenoids are responsible for the red-yellow coloration in autumn leaves, reflecting the longer wavelengths as chlorophyll breaks down due to environmental changes.
Step-by-step explanation:
The pigment responsible for the red-yellow coloration seen in leaves during the autumn color change is carotenoids. These pigments become visible when chlorophyll production slows down due to shorter days and cooler temperatures, which is a signal for the leaves to prepare for winter dormancy. While chlorophyll gives leaves their green color by reflecting green wavelengths, carotenoids absorb in the short-wavelength blue region and reflect longer yellow, red, and orange wavelengths. Carotenoids, residing in the thylakoid membrane, also protect the plant by absorbing excess energy and dissipating it as heat.