The sun emits light in waves of particles that form a spectrum of color (which you can see in a rainbow). The portion of the light spectrum that plants use to grow and to produce blossoms and fruit, the color of light, depends on the plant and its seasonal needs.
Light Color and Photosynthesis
Photosynthesis is conducted by chlorophyll, located roughly in the center of a leaf. Photosynthesis uses energy from light to turn carbon dioxide from the air and hydrogen from water into nutrient sugars. Chlorophyll absorbs and uses all colors of light except green for photosynthesis; so then chlorophyll reflects green, showing us green leaves.
When the days get longer and the days get warmer in the spring and summer, the sun gives off a lot of intense blue light, which has short wave lengths. Trees that were dormant in the winter leaf out, so a new growth cycle can begin. In this phase of their life cycle, plants need lots of blue light to promote the germination of seeds and the growth of seedlings. As the springtime sun grows warmer, plants use blue light to produce buds, blossoms and young fruits.
When the long days of summer get shorter and fruits begin to mature, plants need more red light, which has longer wave lengths. The red light of the late summer sun stimulates plant hormones that regulate the cycle of growth. The plants use this light for flowering and fruiting.
Changing Colors of Plants
We say chlorophyll reflects green, so leaves are green. What happens when the leaves change color?
The changing light of late summer signals to trees that the season of growth is over. Fruits ripen. As summer turns into early autumn, the leaves lose the chlorophyll they needed to grow stems, leaves and blossoms in the spring and produce fruit in the summer. As the chlorophyll disappears, the leaves turn gold, yellow, red and brown.
Plants that go dormant in the cold, red light of winter, wait for the return of the warm, nourishing blue light of spring.
Grow Lights and Light Color
Indoor growers use metal halide and full-spectrum fluorescent light for the blue wave lengths that give their plants a boost when they are growing stems, stalks and leaves. Full-spectrum means that the tubes give off both blue and red light.
To encourage their plants to fruit, growers use high pressure sodium lights that give off orange-red light similar to that produced by the sun as the days shorten. Recently developed LED (Light Emitting Diodes) grow lights, give off two bands of blue light and two bands of red light to meet special plant needs. The pink or violet indoor grow lights you sometimes see are likely LED grow lights.