Fall has always been my favorite season partly because of all the colors. During the spring the daytime starts increasing until the summer months when the sun is out the longest. This sunlight is used by plants to make their food (sugars) through a process called photosynthesis. The main part of this is something called chlorophyll, which makes the leaves appear green. In fall the daytime is decreasing, which then decreases the amount of chlorophyll in the leaves, allowing other chemicals to shine through. Carotenoids (corn and carrots have a lot of this) cause the yellow and orange colors and anthocyanins (red apples and cranberries) cause the reds and purples. Carotenoids are always in the leaf but the chlorophyll dominates in the growing season, so the leaves look green. However, anthocyanins are made at the end of summer due to a particular breakdown of the sugars. Another thing is that certain weather, sunny days and cool (not freezing) nights, can increase the amount of anthocyanins. Wikipedia.org has some cool pictures and more detailed explanations.

That is a really good question! It actually took scientists a few years to figure it out. When plants are growing in the spring and summer, they use light from the sun to make a green protein called chlorophyll. There is a lot of this green protein in leaves, which is why they are green. There are also other proteins that are red, orange, yellow or brown. You just can not see them because there is a lot more green. During the fall, the nights start getting longer so the sun is not out as much. Without a lot of sunlight, plants can not make chlorophyll and so there is not any green color in their leaves anymore. When there is no more green, then you can see the other colors like red, yellow, orange or brown which were there all along! So leaves change color because there is less sunlight in the fall than in the spring and summer.