Leaves are the above ground organs of plants which undergo photosynthesis, the process responsible for the color of the plants leaves. Because photosynthesis takes place in the plant leaves, it is thought that the shape of the leaf plays an important role in how much sunlight the leaf is capable of absorbing, and therefore how much green color is seen. Plants leaves are generally flat and thin helping this absorption of sunlight and for the absorption of carbon dioxide and for the absorption of energy.
Plants contain many substances which are responsible for the colors that they have. Their leaf color comes from the interaction of these substances, mainly porphyrins, carotenoids and flavonoids. The main compound responsible for the color of plant leaves is a porphyrin known as chlorophyll. Plants usually have green leaves because leaves contain chlorophyll, a substance contained in organelles called chloroplasts. Chlorophyll looks green because it can absorb both red and blue light, while the green light of the visible spectrum is reflected. This green light and therefore the green color of the leaves is what humans can see. Interestingly, the red and blue light which is absorbed is what is actually used by the plant to undergo the process of photosynthesis.
Photosynthesis is the process by which plants use the energy in sunlight to change carbon dioxide in the air (along with water), into simple sugars needed for food. Chlorophyll, a pigment that contains magnesium and is essential to the process of photosynthesis, can be found not only in plant leaves, but also elsewhere in the plant.
What about plants whose leaves lose their color in the fall? During the late fall and winter, in certain areas, there is not enough sunlight or water for photosynthesis to transpire. Most trees can live off of stored food from the summer. As seasons change, leaves then “turn off” their food making capabilities. Green chlorophyll then disappears from their leaves and their color starts to fade. This occurs because chlorophyll is broken down at a constant rate and since less is being produced, the green color we originally see starts to fade. So the colors that were there all along ( the reds, and yellows, etc) but, covered by the green from chlorophyll are now visible to the human eye. Carotenoids present in the plant leaves are responsible for the yellow, orange and red colors we often see in leaves, with flavonoids responsible for yellow, red, blue and even magenta colored leaves.