Plants are green because of the chloroplasts inside them. These are the tiny organelles used by the plant to carry out the process of photosynthesis. Chloroplasts are green; therefore, plants are green. Of course, the question then becomes: why are these chloroplasts green? The answer is chlorophyll--the pigment inside the chloroplasts. It absorbs red and blue light from the sun and uses this light to carry out photosynthesis within the plant.
After discovering that chlorophyll is the initial reason behind the green color, we must then question why chlorophyll itself is green, as opposed to blue or orange or any other color across the spectrum. To answer this question, we have to delve into the science behind pigments. Pigments absorb light. This is their function. We have established that chlorophyll absorbs red and blue light. So why, then, does it make the plant green? This is because a pigment becomes the color it does not absorb. In the true light spectrum, there is only red, green and blue. Chlorophyll absorbs the red and blue, and reflects the green. Thus, the plant appears green.
Not all plants are green, at least not all the time. In the fall, leaves change color and can turn a tree-lined street into a virtual wonderland of various shades of orange and yellow and red. Why does this happen? Because chlorophyll is not the only pigment present in a plant. There are also red pigments and yellow pigments, known as anthocyanins and carotenoids respectively. These pigments absorb green light and reflect red, or reflect just enough red and blue to create a yellowish orange. Not only do these pigments become prominent once the photosynthesis in the plant goes dormant in the fall, but they can affect the plant's color throughout the year, creating varying shades of green.
Photosynthesis, in its most basic terms, is the process by which a plant converts light into sugar. A common misconception is that a plant needs the heat and light of the sun to grow, but this is not true. A plant merely needs light of some source to carry out this function. A look at any green indoor plant will confirm this basic principle. Of course, on the other hand, many plants need much more light than can be delivered by the usual indoor fixtures, which is why they are primarily outdoor plants. Almost all photosynthesis occurs in the leaves of a plant, with little to none of this activity originating in the stem.
Color is an important aspect of determining what plants are harmful to humans, as well as other animals. Naturally, some of these colors spring not just from the plant itself, but from the flowers which it produces. Take the narcissus plant, for example. It has a green stalk, leading up to a flowery, bright yellow bulb. Some people call these plants daffodils. Their appearance causes some to mistake them for onions, but they are unsafe for human consumption. The purple rhododendron is another flowery plant that produces a honey that is toxic and can even lead to death.