The vast majority of plants are green and are so for a simple and specific reason: chlorophyll. But, there's more to it than just that. To truly appreciate why plants look green to us, it helps to understand a little more about how human vision works, how plants work and how light itself works.
Our eyes can perceive all colors within the visible spectrum. If you look at a rainbow, which shows all of the colors of visible light, you will notice that red and blue are on the outside of the rainbow with the other colors, including green, in the middle. The eye recognizes these colors by detecting the wavelengths of light that an object reflects.
Photosynthesis is the process by which plants convert energy from light and water into sugar that is used for the continued metabolism and growth of the plant. Light is necessary for photosynthesis to occur within the leaf. Many cells within the plant, called chloroplasts, carry on this process. Within each chloroplast is a chemical pigment known as chlorophyll.
Chlorophyll acts as a filter, allowing the plant to absorb the proper wavelengths of light for photosynthesis to occur. The chlorophyll in plants absorbs the red and blue wavelengths of light. It reflects what it does not use, which is primarily the green wavelengths.
Because plants reflect green wavelengths of light and absorb all others, when we look at the plant, we perceive the color of the plant as being green. In parts of the plant where there is no chlorophyll, such as in flowers, other colors may be visible.
During winter, many plants, such as maple trees, drop their leaves. Before that occurs however, the color of their leaves change from green to bright yellow or red. As the temperature drops, changes in the plant cut off the flow of sugar into the leaves in preparation for winter. The chloroplasts will begin to die and the chlorophyll in them will break down. When that happens, the green light will no longer be reflected and other pigments in the leaves, such as carotenoids, will reflect other colors.