I think the tree changes colors because they need a way of surviving the winter. The pigments protect the leaves from getting "frostbite". They have pigments such as chlorophyll (green), carotene (orange), xanthophyll (yellow), and anthocyanin (red).
The search feature above is actually quite good for finding what you need! Type in the one or two words that you are looking for information on, and it will generate a list of every page that mentions that word.