Background research

A leaf contains mainly three leaf pigments: chlorophyll, carotenoids and anthocyanin. 

Chlorophyll is involved in photosynthesis, which is the process of making food by converting carbon dioxide and water into sugar and oxygen with the aid of sunlight and chlorophyll. Chlorophyll gives the leaves a distinct green colour. 

Carotenoids are present with chlorophyll and organelles called chloroplasts, where photosynthesis occurs. When a leaf dies, the carotenoids are left over in the chloroplasts and revealed from the loss of chlorophyll. 

Anthocyanin are water-soluble pigments produced via the flavonoid pathway in the cytoplasm of the coloured plant cell. The attachment of the sugar molecule makes them particularly soluble in the sap of the vacuole, where these molecules are stored once they are launched.

Chlorophyll is so bright green that it blocks the yellow, orange colour of the carotenoids. In the growing season, chlorophyll is broken down and reproduced. But when it starts dying, nutrients and water does not reach the chloroplasts and hence chlorophyll is not reproduced. That means chlorophyll is only broken down but not reproduced so the amount of chlorophyll decreases. This allows the yellow and orange colours of the carotenoids can show through. In contrast, anthocyanin is not present in the leaves initially but it is actively produced as a leaf dies right before the leaves fall of the trees.
The red pigments are produced so that the trees will know that the leaves need to fall. This is essential because the remaining healthy leaves will have more chance of getting sunlight.