Alright well the cooler weather is arriving up in the northern states (sadly not in warm Florida) and it's time for the leaves to change color and fall off the trees. I never really wondered though, why they change their color.
Well hopefully you know that leaves get their green color from the chlorophyll is the dominant pigment that makes food and it gives the leaf a green color. Other pigments and carotenes also are in a leaf with orange or red or purple or yellow colors. The green though is the dominant color and is the only color shown.

During fall though the change in length of daylight, and change of temperature the leaves stop the food making process and chlorophyll breaks down allowing the other colors to show. Other chemical processes may start up giving a different color as well. Because the food process is stopped the leaves fall off eventually.

Source: http://www.esf.edu/pubprog/brochure/leaves/leaves.htm

So it is kind of like the bear, the tree goes in hibernation until spring arrives. Because there wouldnt be much sense into supporting the leaves when they will not produce enough food for them I guess.

Sorta like animals that may come out for particular seasons... they dont come out untill a particular season arrives. Same may apply for myths or kids such as snowman only coming out in winter, or a similar fictional kids story about a "snowman" son