Blueberry Leaves

Why do leaves turn colors before dropping off in the fall? These are blueberry leaves (genus Vaccinium) . They're red where they received sun, but yellow where they were covered by another leaf.

Maybe the red is a pigment that helps with photosynthesis, but also works as a sunscreen. Perhaps the green pigments are able to be reabsorbed by the plant, but the reds aren't. But if during the summer both pigments are in the leaf, how does the green cover up the red? Perhaps the green changes to red in the fall.

I wonder what they looked like during the summer.