I had noticed the differences in leaf color on the same tree before but it never struck me as much as this fall. The Bradford pear and sweet gum trees are especially vibrant this fall (2011) in the Northeast US. The leaves that receive the most sunlight show more red than leaves on the north side of the tree or shaded by other leaves. The effect can be a two-toned tree when viewed from the east or west. Here are a couple of shots of sweet gum leaves that illustrate how different the leaves on the same tree can be: