Leaves have their color due to pigments. The color they have is due to the consistent photosynthesis process that takes place.
However, during the season of fall, the leaves end up having lack of required sunlight. This results in incomplete process of photosynthesis.
This lack of sunlight causes change in the color of the leaves and as a result the leaves fall off the trees and tips.
Hence the season of fall is observed.
I hope this answer helps!😊