Plants are green because they contain a substance called chlorophyll that is green in color. Chlorophyll is found in the part of plant that is called the chloroplast. It is a compound that absorbs sunlight. Chlorophyll uses the energy from the sun to manufacture food for the plant, using the nutrients from the soil water and carbon dioxide.
This process is known as photosynthesis and Photosynthesis is possible only if chlorophyll is present. Without the green colored chlorophyll, a plant cannot manufacture its own food and would die. There are actually other colors found in a plant too, but they cannot be seen because chlorophyll is a strong absorber of light. Light is made of many colors in different wavelengths and chlorophyll absorbs all the other wavelengths except green making the plant appear green to us.