UC Davis Horticulture and Agronomy Faculty Astrid Volder provides an easy-to-digest explanation about why leaves change color in fall.

Why Leaves Change Color in Fall: The pigments that determine leaf color change in the fall are always there, but mostly hidden as large amounts of continuously generated chlorophyll (the green pigment) dominate during the growing season.

What Happens as Fall Approaches: When fall starts, the production of chlorophyll stops and chlorophyll that is present begins to break down. This allows the other pigments, that were always there – like carotenoids, which make yellow and orange colors – to become more visible.

Why Leaves Don't Fall Off Right Away: The process of leaves changing color and falling off takes a few weeks. During this time, trees take back important nutrients, mainly nitrogen, from the leaves. To support this resorption process,  leaves need to stay active and keep performing photosynthesis.

The Problem with Bright Sun and Cold Weather: When there's lots of sunlight and it's cold, it can be tough on leaves. Their ability to perform photosynthesis slows down and high light conditions will damage them.

How Some Trees Protect Themselves: Some trees, especially those from cold and bright areas like the mountains, make a red pigment to protect the leaves from high light damage called anthocyanin. This causes the vibrant red leaf colors often observed after the first frost. 

Why Cloudy Days Affect Leaf Colors: When the sky is cloudy, it means less sunlight reaches the leaves. This can make the red colors less intense because less anthocyanin is produced.

Temperature's Role: Cold temperatures make fall colors look better while warm temperatures make the chlorophyll break down faster and the leaves fall off sooner, so we enjoy the colorful leaves for a shorter time.

To recap, leaves change color because of hidden pigments, and different factors like sunlight and temperature affect how vibrant those colors become in the fall.