Chlorophyll breaks down, revealing the yellow pigment that was there all along. But red? Red is a different story altogether.

This is the time of year when Nicole Hughes gets constantly distracted by the fall foliage as she drives around North Carolina. "I'm always looking," says Hughes, a biologist at High Point University.

The red pigments in some fall leaves have proven to be a puzzle for researchers who debate why leaves bother to go red. Why do only some leaves turn red in the fall? Scientists can't agree on an ...