The reason as to why many plant species, including most grasses, are green is the presence of chlorophyll. This is a bright pigment that absorbs blue and red light, while reflecting green light. Blue light has high frquency and short wavelength, while red has low frequency and longer wavelengths. Reflection of green light by chlorophyll is what accounts for the green color in grass and other plants.
The reason why objects appear to be of given colors is as a result of reflecting that color, while absorbing all other colors. Yellow things are yellow because they reflect yellow and absorb every other color. Pigments are the molecules responsible for absorbing colors. Both plants and animals have pigments.
Although plants have several kinds of pigments, chlorophyll is the most predominant. Chlorophyll reflects green light and absorbs others. For plants to be able to carry out the process of photosynthesis, they need to absorb light. Energy from light is used in converting carbon dioxide into carbohydrates and sugars.
Quanta of light absorbed by chlorophyll are transferred to some special molecules as energy. Stimulation of these molecules sparks an electron that triggers chemical changes within the plant. Chemical energy is further processed into sugar.