An interesting article from Not Exactly Rocket Science with a different theory as to why some leaves turn red and purple in the autumn.
"Autumn is a time of incredible beauty, when the world becomes painted in the red, orange and yelllow palette of falling leaves. But there may be a deeper purpose to these colours, and the red ones in particular. In the eyes of some scientists, they aren't just decay made pretty - they are a tree's way of communicating with aphids and other insects that would make a meal of it. The message is simple: "I am strong. Don't try it."