From NPR.

This is a busy time of year for scientists who are trying to understand autumn leaves. The most mysterious color is red.

In the fall, trees take apart their leaves’ green chlorophyll to recover nitrogen, revealing yellow pigments that were there all along. But red pigments are different — trees make brand new red chemicals just days before a leaf is about to fall.

Why bother creating costly red pigments in leaves that are almost done for? Scientists still aren’t sure.