According to Oregon State University's Extension Service, quality of light refers to the wavelength, or color of the light. Sunlight appears white because it produces light across all colors of the visible light spectrum.
OSU also states that light from the blue part of the visible light spectrum is responsible for promoting vegetative growth in plants. Blue light with wavelengths between 430 and 470 nanometers is ideal for vegetative growth.
Red light, combined with blue light form the visible light spectrum will encourage flowering and budding in plants. Red light with wavelengths between 630 and 700 nanometers is ideal for flowering.
Plants appear green because they do not absorb green light, they reflect it. Green light is the least effective quality of light for promoting any type of plant growth.
There are several light bulbs on the market that are ideal for encouraging plant growth. Fluorescent light bulbs and metal halide bulbs are ideal for vegetative growth because of the bluish-white light they produce. High-pressure sodium bulbs are ideal for encouraging flowering because they produce an orangish-red light.