Leaves get their green color from a chemical called chlorophyll. Chlorophyll is what helps plants to convert water, carbon dioxide, and sunlight into glucose which is what the plants eat. During fall the days get shorter and light has to travel further to get to the earth, because the tilt in the planet, the plants can no longer get enough sunlight to perform photosynthesis so they begin to go into a dormant state. As photosynthesis stops the chlorophyll in the leaves begins to break down. Other chemicals that are present in many leaves, like carotenes and xanthophyll, take longer to break down then the chlorophyll. So once the green chlorophyll is gone then the reds, yellows, and oranges of these other chemicals are all that remains until the leaves fall from the tree and end up floating alone at the shore of the river.