Why do leaves change color in the fall?

The yellows and oranges are not new; they were there all summer. Leaves are packed with chlorophyll, the green pigment that runs photosynthesis, and it is so abundant it masks the yellow and orange carotenoids sharing the leaf. As days shorten, trees stop maintaining chlorophyll, it breaks down, and the hidden pigments finally show. So autumn yellow is less a change than a reveal.

Red is the exception: trees manufacture red anthocyanins fresh in fall, from sugars trapped in the leaf as the tree seals it off, at some real energy cost. Why they bother is still researched, with sunscreen-for-the-recycling-process leading the theories.

This is also why the display varies by year. The best color follows warm sunny days and cool, not freezing, nights, which maximize trapped sugar and thus red production. Drought and early hard frosts mute the show, and the US Forest Service issues fall foliage forecasts on exactly this science.