In the animal world there are many different colours. There are orange and brown giraffes, white polar bears, blue beetles and red and yellow birds. But in the plant world, the leaves of nearly all plants are just one colour - green. Why?
The biggest difference between plants and animals is that animals eat other things for their food and plants don't. Plants are able to make their own food. Leaves have a wonderful substance inside them that makes food out of air and water, with the help of sunshine. This substance is called chlorophyll. And chlorophyll is green.
So a healthy leaf is green because it is filled with chlorophyll. Leaves which don't get enough light turn pale and yellowish because the chlorophyll can't work without sunlight.
(World Book Encyclopedia: Green Kingdom)