Visible light with wavelengths between 430 and 460 nanometers (a nanometer is one-billionth of a meter) appears blue. According to the Oregon State University Extension Service, blue light is used by plants to encourage vegetative growth.
Red light is also very important in the development of plants. Visible light with wavelengths between 650 and 700 nanometers appears red. The extension service claims that red light is ideal for encouraging budding and flowering in plants.
Metal halide light bulbs, high-pressure sodium light bulbs, fluorescent light bulbs and LEDs (light-emitting diodes) are often employed by consumers to provide lighting for indoor plants.
Light intensity is also important in promoting indoor plant growth. "Lumens" is the term that has been designated to measure light intensity of light sources. Metal halide and high-pressure sodium light bulbs are often used in greenhouses and other indoor applications involving growing plants because they produce generous amounts of lumens--250-watt metal halide bulbs produce around 58 lumens per watt of power drawn, while high-pressure sodium bulbs produce around 87.
White light is light that includes all colors of light in the visible light spectrum. Fluorescent light bulbs and metal halide light bulbs emit whitish light that includes blue and red light. For this reason, they are often used for indoor plant lighting. High-pressure sodium light bulbs emit more of a yellow, orangish-red light, which is why they are often employed to encourage flowering in plants.