As you know, plants get their energy from light, but you might not know that different colors of light have different energy levels.

Think about the colors of a rainbow. All of these colors are in white light. If you use a prism, you can separate the white light into the colors. (Thats what the raindrops are doing when we see a rainbow.) The colors are always in the same order. The NASA site has a nice picture of this so that you dont have to try to remember where the colors are:

The highest energy light is at the purple or violet end of the scale. It has short wavelengths. The red end is lower energy, it has long wavelengths. The shorter the wavelength the more energy a color has. Imagine dribbling a basketball. You can dribble slowly (long wavelength) or really fast (short wavelength). It takes more energy to dribble fast.

Now heres the next wrinkle in the story. The color you see when you look at an object is the color that light is reflecting back at your eye. Plant leaves (usually) look green to us because they are reflecting the green back. This means that they are not absorbing the green light. They are absorbing the other wavelengths. The ones that give the plant the most energy are the ones at the purple end of the spectrum. They still get energy from the red end, but not as much.

What do you think would happen to a plant that got only green light?

Plants are certain colors because their pigments absorb light most effectively in certain regions of the visual light spectrum. As you know, plants are usually green, which means that most other colors are absorbed. One of the most common pigments is called chlorophyll, and one of the varieties is responsible for the green color of plants; it strongly absorbs blue and red light, which leaves only the green light to make it to our eyes.

Plants have more than one pigment, so they can capture different regions of light, but not all pigments are made equal. Chlorophyll is the most effective pigment, which is why most plants have it. Based on this, you would expect that green light wouldn't be the best for growing plants in the long term.