Like many plants, most species of grass produce a bright pigment called chlorophyll. Chlorophyll absorbs blue light (high energy, short wavelengths) and red light (low energy, longer wavelengths) well, but mostly reflects green light, which accounts for your lawn's color.
But chlorophyll isn't just for eye candy. It also figures importantly in the process of photosynthesis, by which plants convert an inorganic material (light) into a useable, organic one (sugar). Chlorophyll molecules absorb quanta of light and transfer the energy to special molecules that can, when stimulated, fire off an electron that causes chemical changes in the plant. Further processes turn the chemical energy into sugar.
While grass can just sit there and grow, we poor animals have to hunt down our food, which may seem a trifle unfair by comparison. But then, the grass is always greener …