Photosynthesis takes places over a series of physical and chemical mechanisms divided into two phases: the light reactions and the dark reactions. During the light reactions of photosynthesis, light from the sun is captured through a series of reactions involving the chemical chlorophyll. This results in the synthesis of two high-energy chemical compounds: ATP and NADPH, whose chemical energy is held by electrons that can be transferred easily to other compounds. The light reactions require water (H2O), from which oxygen is released during the process. ATP and NADPH are used subsequently to produce glucose from CO2 in the next phase of photosynthesis, the dark reactions.
Pigments are chemicals which reflect certain wavelengths (colors) of light but not others. Different pigments reflecting different wavelengths give flowers a variety of colors. Additionally, seasonal changes in the relative synthesis of different pigments accounts for color changes in leaves during the autumn.
Pigments are vital components of the machinery of photosynthesis, the most important pigment being chlorophyll. Chlorophyll is a big molecule which captures energy from sunlight and converts it to high-energy electrons. This happens during the light reactions of photosynthesis. Pigments other than chlorophyll include carotenoids (which are red, yellow and orange) and phycobilins. Phycobilins include phycocyanin, which imparts a bluish color to blue-green algae (also known as cynanobacteria), and phycoerythrin, which gives a reddish color to red algae.
A pigment imparts a specific color to a plant by reflecting a particular wavelength, or color of light. When a certain color of light is reflected, it is not absorbed to energize a pigment to drive photosynthesis. Since chlorophyll is the most important pigment and reflects, and therefore does not utilize, green light, green is the color of light that is least effective in driving the photosynthetic process.