In late summer, as daylight hours shorten and temperatures cool.
Carotenoids are present in leaves the whole year round. But their orange-yellow colors are usually masked by green chlorophyll.
The reds, the purples, and their blended combinations that decorate autumn foliage come from another group of pigments in the cells called anthocyanins.
The brown color of leaves is not the result of a pigment, but rather cell walls, which may be evident when no coloring pigment is visible.
Autumn produces some of the most vibrant colours of the year in the leaves of deciduous trees and shrubs. However, colour can vary from year-to-year, and between individual trees and shrubs of the same species. This can be due to reasons such as the weather.