What triggers tree leaves to begin changing color in the fall?

Chlorophyll breaking down

Explanation:

I found this science text, I hope it helps.

"But in the fall, because of changes in the length of daylight and changes in temperature, the leaves stop their food-making process. The chlorophyll breaks down, the green color disappears, and the yellow to orange colors become visible and give the leaves part of their fall splendor."

Essentially, leaves change color because of changes in temperature and daylight in fall / autumn, causing the chlorophyll inside the leaves to break down and make the leaves warm colors like red, orange and yellow.

If I am wrong, I apologize!