Ive searched the archives high and low, but no sign of this one being answered by Mr. Adams. He must have answered it, considering what an obvious question it is, but perhaps someone here can help out:
What causes leaves to change color in autumn?
But, as noted in the first paragraph, the anthocyanins are not present in the leaf until the autumn, after chlorphyll production ceases. So the reds in the maples and other bright colors of some trees were not there underlying the chlorphyll, but produced after production of chlorphyll has stopped. BTW, this was discussed by Adams (but described incorrectly) and also in a prior thread in this forum.
It is also important to note that the reduction in hours of daylight as well as temp triggers the reduction in sap production. This, in turn, cuts down on the production of chlorophyl which allows other color-producing agents, such as xanthophyl to dominate bringing out yellow hues, for example. (As best as I recall from my past advanced biology knowledge). - Jinx