Photosynthesis  and Color (Photosynthetic spectrum) 

In nature, plants depend on the energy of the sun. Through a process called photosynthesis, sunlight is converted into sugars to provide fuel for the plant’s growth. These sugars are utilized as needed in a process called respiration, and excess sugar is also stored for later use. Photosynthesis is made possible by chlorophyll, which is contained within the leaf cells. Chlorophyll gives vegetation its characteristic green color. Light is trapped by the chlorophyll, activating the process of photosynthesis. 

Inside the chlorophyll, light energy is combined with carbon dioxide and water to produce oxygen and sugar. The sugar is then oxidized (or metabolized) through the process of respiration, producing carbon dioxide, water, and energy for growth. Excess oxygen and water are transpired by the leaf into the air. Plant growth, therefore, is directly affected by the color, intensity and duration of the light the organism receives.

Color (Photosynthetic spectrum) 

Photosynthesis is most pronounced in the red (600-680nm) and blue (380-480nm) wavelengths of light.Blue light stimulates hormones that trigger growth and inhibit dormancy. Blue light powers photosynthesis causing tips to grow towards the source (phototropism). Metal Halide lamps emit strong levels of blue light making them good for promoting the growth of leafy plants. Blue light also serves to keep plant growth compact and shapely by minimizing the distance between internodes (branches). Green light is reflected, that is why plants appear green, however some green light is required for growth. HID lamps do not emit much green light, neither do high pressure sodium lamps. Red light also powers photosynthesis, aids in seed germination , helps to form pigments and aid flowering. Red light is also responsible for triggering dormancy in some plants. High Pressure Sodium bulbs emit red light and are generally better for flowering and fruiting plants.Far-Red light speeds up some full sun plants, reverses some red light effects. HID lighting usually doesn’t emit far-red except in the case of some High and low pressure sodium bulbs, more so in the form of heat rather than photosynthetic light.