Green leaves are green because chlorophyll absorbs all of sunlight's rainbow of colors except the green part, which is reflected.

You know that too much sunlight can damage your skin, fade colors in clothing and cause other problems. Therefore it shouldn't be surprising to find out that sunlight also causes chlorophyll molecules to break down. In fact, during the summer, green plants must continually create new chlorophyll to replace what has been destroyed. This creation, or synthesis, of chlorophyll requires not only sunlight but also warm temperatures. For this reason you can see why fall's cooler weather encourages our trees' leaves to begin showing colors other than green.

But, where do those other colors come from?

There's another substance in many leaves known as carotene. Carotene is a kind of "chlorophyll helper." That's because carotene absorbs sunlight energy like chlorophyll, but instead of keeping that energy and conducting photosynthesis with it, it passes its energy on to chlorophyll which then uses that energy to perform photosynthesis. Carotene is known technically as an "accessory absorber." Carotene holds up much better under sunlight than chlorophyll, so often in the fall when chlorophyll disappears from leaves, carotene is left behind. Since carotene absorbs blue-green and blue light in sunlight, the light it reflects back to our eyes from leaves in which the chlorophyll has disappeared is

http://www.backyardnature.net/lf_color.htm

Chlorophyll gives plants their green colour. There are other pigments in the leaves too, such as xanthophylls (yellows) and carotenoids (yellows, oranges and reds). These pigments are also used in photosynthesis but occur in lesser quantities than the green chlorophyll. The combinations of the different pigments make different shades of green.

Now the reason that plants look green is that they are trying to obtain energy from the sun using a particular part of the light spectrum, mainly the red and infra red wavelengths. If you remember from your physics classes the colour you see is the colour that is reflected from the object, the other colours are absorbed. So in the case of green plants, the green wavelength is reflected and all the other colours, especially reds and blues, are absorbed to drive the energy cycle in the plants.