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Color and Wavelength
Visible light spans the spectrum from violet, with a wavelength of about 400 nanometers, to red at 700 nanometers. Bright sunlight contains all the wavelengths between these markers in about equal quantity, which is why you perceive its color as white. However, plants do not utilize all colors equally. Leaves generally reflect green light while absorbing red and blue to use in photosynthesis.
Photosynthetic Pigments
Plants contain several different pigments which are involved in photosynthesis. Each pigment evolved for its own purpose, and thus prefers slightly different colors of light for maximum photosynthetic efficiency. Chlorophyll A, the most abundant of the photosynthetic pigments, experiences peak light absorption in blue light. Strong chlorophyll A activation is related to leaf growth. Chlorophyll B, the second most common pigment, absorbs both blue and red light. Chlorophyll B is linked to flower growth, as opposed to leafy growth. Other pigments, such as the carotenoids and xanthophylls, play a lesser role in a plant's normal functioning. They expand the range of absorption to include some green and yellow wavelengths.
Canopy Light
When a plant is in shade, it is not exposed to the direct light of the sun. Instead, light hitting the leaves consists of ambient light bouncing off the nearby environment as well as light passing through the object casting the shadow. This causes a decrease in the quantity of light, but it also may affect the quality of light. In the natural world, shade is often the result of a leafy canopy over the plant. As a result, light passing through the leaves of the canopy will appear green, just as the leaves themselves do. Plants under a canopy suffer from decreased efficiency due to the increased proportion of green light that reaches them. If all other factors are equal, including temperature, the canopied plants will grow less than neighboring plants in direct sunlight.
Artificial Light
Man-made light bulbs produce incomplete spectra that do not fully recreate the sun's light. Fluorescent lights produce many wavelengths in the blue and yellow range of the spectrum. While the yellow light is not useful to most plants, the large amount of blue light helps plants and seedlings produce leafy growth. Incandescent lights are fairly poor growth aids for most plants. They produce mostly orange and red wavelengths, with very little blue. The light emitted by incandescent light bulbs is accompanied by large amounts of infrared radiation (heat).