Light quality refers to the color, or wavelength, of light that's emitted from any light source. Phytochrome is a family of proteins within each plant that allows them to sense the colors of light. Sunlight contains the full spectrum, or wavelengths, of light that plants need to complete photosynthesis. Photosynthesis is the process by which plants transform light energy into food and oxygen.
A prism will break the spectrum of color from the sun into bands of red, blue, indigo, violet, green, orange and yellow. Even though plants use all colors of the light spectrum, red and blue are the two primary colors necessary to complete the energy conversion. The amount of red and blue light within a light source will affect plant growth in different ways. Green is reflected rather than absorbed by plants, which is why they appear green.
Blue light regulates the rate of a plants growth and is especially helpful in plants with lots of vegetation and few to no flowers. Blue light regulates many plant responses including stomata opening and phototropism. Stomata are openings on or beneath the surface of the leaves.
A plant's moisture loss is primarily due to the stomata and blue light controls the degree of stomata opening, therefore blue light regulates the amount of water a plant retains or expels. Phototropism is the definition of a plant's response to light; the stems grow up toward the light and the roots grow down, away from the light. Metal halide grow lights emit more light in the blue spectrum and are the best source of indoor lighting to use for plant growth if there is no sunlight available.
Red and orange light triggers hormones in plants that increase flowering and budding, but plants cannot grow with red light alone. They also need blue light to help regulate other types of responses. Red light stimulates flowering and foliage growth, but too much red light will cause a plant to become spindly.
HPS (high-pressure sodium) grow lights emit a red orange glow and are excellent companion lights for growing conditions that include some natural sunlight or other light sources with high levels of blue light. Red light induces germination and blue light promotes seed growth, but far-red light inhibits germination.