Plants usually have green leaves due to the presence of the pigment, chlorophyll. They appear green because this pigment has the unique ability to absorb all the colors in the spectrum, apart from green light, which it reflects (and we see).
Light containes many wavelengths of light energy, from dark blue through to palest yellow and you can separate these with a prism into their component colors. The energy within light is used by green plants in a unique way. It excites electrons within the plant cells, in special organelles called chloroplasts. These use the energy contained within light to convert the elements of water and carbon dioxide into sugars. Water is gained from the soil through root hairs and cabon dioxide from the air through stomata, which are tiny pores intheleaves. The plant uses the sugars produced as an energy store and as the building blocks for other chemicals such as enzymes, proteins and lipids which it uses for all metabolic processes within its tissues.
Green plants use all the light energy range apart from green light, which chlorophyll reflects but not all plants are green. Some have flavinoids and caratenoids which are red, brown or orange and these plants use different light wave lengths. These plants are found in places where ordinary light wave lengths do not penetrate such as in water, at greater depths and in dark places. Examples include sea weeds, some algae and mosses.
Because our eyes see light in a certain way, we see the leaves of most plants as green but not all animals do. Some animals like bees and other insects see light wavelengths differently. Bees, for example, view the world in an ultraviolet spectrum so they may see 'green' plants as many different colours according to whether the range of pigments they contain reflect or absorb ultra violet light. This can be of importance to a bee looking for pollination paths, which are shown on many plants including digitalis (fox gloves) and can be seen using ultra violet light as a 'pathway' leading to the nectaries. So, the bee follows the path to the nectar, while at the same time, being led by the path to pass the stigma and anthers to pollinate the flower. A very clever and effective adaptation of both plant and insect.
Of course, we do not see all green plants in the same shade of green. This is because, even within the green light wavelength, there are a range of different energies and the pigmentation of plants has adapted to be available in different quantities according to whether the plant grows in shade, sun or semi-shaded areas naturally. By absorbing only the energy which is useful to it and reflecting the rest (so some reflect lighter shades of the green spectrum and others the darker shades) they make maximum use of the energy available.
it is a continuous wonder tha tplants are so beautifually adapted to their conditions and environment and that the creatures which depend on them adapt as well. We are continually learning more about our green leaved friends (and blue, red and brown ones) and out reliance upon plants as providers of food, materials and color is absolute.