Leaves change color in the Fall because chlorophyll (a pigment that makes them green) breaks down. Chlorophyll is a chemical that uses sunlight energy to transform carbon dioxide and water into carbohydrates that feed trees during Summertime & Spring. This process is called photosynthesis.
Trees get all the nutrition they need and when temperature drops & daytime becomes shorter, chlorophyll breaks down showing the dominance of other chemicals found in leaves: xanthophyll (yellow colors) and carotene (orange colors). Some trees also produce anthocyanin (red pigments). Crisp (but not freezing!), sunny days during Fall, boost production of anthocyanin. You may not see as many red leaves if temperatures drop below zero, or if it's too wet and cloudy.