Bob, Grass and most other plants are green because they contain a pigment known as chlorophyll. The chlorophyll is used in the process of photosynthesis where a plant produces sugar in the presence of sunlight.
Some examples are the fungi which decompose dead, and sometimes living, tissue, for their food.
You will find that a green plant needs light to make food. If the source of light is cut off, the plant dies. The process of photosynthesis is described in great detail in many science books. It is really the process by which life as we know it is able to continue and renew itself. Argonne National Laboratory, Division of Educational Programs, Harold Myron, Ph.D., Division Director.