Why do some trees turn brilliant colours in autumn?

A change in leaf colour is triggered by cold weather in autumn. The basics are plants stop producing chlorophyll (which is green), breaking down and using up the remaining chlorophyll in its leaves. This exposes the underlying pigments of yellow and red. However, it's a complex subject. Check out the fascinating read on research into autumn leaves (Why Does Fall Foliage Turn So Red and Fiery? It Depends) on the New York Times website (www.nytimes.com)