Light guides many of the growth patterns and behaviors that you observe in your plants. When houseplants grow toward a bright window, when flowers blossom at the same time each year and when seedlings kept in a dim room grow spindly and fall, you are witnessing the remarkable effects that light has on plants. In particular, colored wavelengths of light guide plant behavior.
Light occurs in wavelengths, with longer wavelengths corresponding to red light and smaller wavelengths forming blue- and violet-colored light. To your plants, the red and blue wavelengths are most important, guiding flowering and growth. Paradoxically, plants don't use green wavelengths of light; in fact, plants appear green because they won't absorb these useless green wavelengths, throwing them back and making the plant look green.
Different colors of light affect plant growth differently. Red light encourages flowering, according to the Oregon State University Extension, while blue light increases the vegetative growth of stem and leaves.
A substance called phytochrome regulates red light-dependent responses in plants. Phytochrome acts as a timekeeper for plants, as well as helping them to detect lighted areas from shady spots or areas receiving only low light, according to the Missouri Botanical Garden. Two types of phytochrome change form depending on the type of red light they receive or their exposure to darkness, providing plants with a means to measure the length of the day or to know how much light is present in their environment. When the balance of the two types of phytochrome is correct, flowering plants will blossom. Phytochrome explains how individual species of plants always flower at the same time of year.
Phototropins govern plant responses to blue light. When you observe houseplants bending toward a light source, you are witnessing the effects of blue light and phototropins. When phototropins detect blue light, they inhibit growth of that part of the plant. The darkened part of the plant continues growing normally, with the result that the darkened side elongates faster, causing the plant to grow toward the light and maximize its exposure to light.
Understanding how different wavelengths affect growth and flowering allows you to better control these behaviors using light. For example, as the Oregon State University Extension points out, fluorescent lighting rich in blue light encourages houseplants to grow and seeds to germinate. Adding a bit of red from an incandescent light encourages your indoor flowering plants to blossom, if the timing is right.