A change in leaf colour in autumn, from green into other colours, such as yellow, red or orange. This is common in many trees and can be a spectacular view when it happens simultaneously over large areas. It may also seem meaningless.
Why use resources to change the colours of the leaves that are about to die anyway?
The answer is largely that as autumn proceeds, it becomes too costly for the plants to keep their green colour. What happens is actually that the leaves stop producing the green pigment (from chlorophyll), not so much that they start producing new colours. Most of the bright yellow and red colours are in fact present in the leaves during summer too but masked by the green. The colour changes also have important functions that prepares the trees for their winter dormancy.