The substance that gives plants their green color is chlorophyll. This pigment absorbs mostly blue and red light, but reflects green light. It is the substance that allows plants to perform photosynthesis, using the light it absorbs to power the process of transforming water and carbon dioxide into carbohydrates.
There are actually two types of chlorophyll found in plants, but both are green. They absorb slightly different wavelengths, however, and allow plants to use more of the light they receive than they could with just one type. Chlorophyll is a very strong pigment, but leaves often have other pigments in red and yellow that are revealed when chlorophyll breaks down.