If efficiency and simple cause-and-effect were in charge of things, we wouldn't have autumn's colors. Trees are models of efficiency in most of their processes and activities -- the photosynthesis that goes on in their leaves, their unique system of drawing water and nutrients from the ground through roots and turning it into sap, which is performed without waste.
But in technical terms the color that comes to autumn leaves is waste, sheer excess and leftover. It is created by substances revealed only when the tiring tree seals off the sap circulation and no longer charges up the chlorophyll in the leaves. The old clorophyll disintegrates and yellow pigments called carotene and xanthophyll become visible. The reds and purples appear when the sun has oxidized sugars and acids the tree has abandoned in the leaves. When the leaves have passed their peak in color and fall from the trees, they molder into humus which eventually will feed the parent tree. But the color adds nothing to the humus.
Fortunately, for us, there is no efficiency expert or stern taskmaster that abhors superfluous qualities monitoring the trees. As a result, we get fall's magnificent coloration.