There are two major hypotheses to explain colorful fall leaves. One is that
red anthocyanins act as a sunscreen and act as antioxidants. The other is that
colorful leaves repel insects.

In that case the trees would have red leaves in midsummer, when there is more
sun and there are more insects.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"When you come to a fork in the road, take it." Yogi Berra