It's a problem designers face all the time: you spend hours carefully matching colors in various environments. Then, when you bring them all together in the same place, they don't match. What happened? In a word: metamerism.
Metamerism is defined as the difference in appearance of two colors when viewing those colors under different light sources. Simply put, colors that may appear to match when viewed under one type of light source such as a light bulb, appear to not match when viewed under another type of light source such as fluorescent light or daylight.
How do we perceive color?
If you sit in your living room at night with the lights turned off, you do not see any color. In fact you don't see anything. The room is pitch black. When you turn the light on, you see the color of the objects in the room that are within your field of view. What's happening?
Paint under daylight (left) and incandescent light (right), tinted with different colorants. Paint under daylight (left) and incandescent light (right), tinted with the same colorant.
The lamp in your living room emits energy from the filament in the bulb. The specific wavelengths of light energy depend on the type of bulb. Different types of bulbs emit energy of differing wavelengths. These wavelengths are of a frequency that your eyes can respond to and interpret as color.
What happens when we see colors?
When you turn on the lamp, the energy emitted by the bulb strikes the objects in the room. The dyes in the fabrics, the pigments in the tiles, and the color pigments in the paint absorb specific wavelengths of the light energy, depending on the molecules of the color pigments in these materials. The color pigments also reflect specific wavelengths of light energy. The reflected visible-spectrum energy enters your eye and sends signals to your brain that you interpret as color.
So where does metamerism come in?
In the same way that objects absorb and reflect specific wavelengths of light energy, the lamps, bulbs, and sun emit various wavelengths of light energy within the visible spectrum.
The sun emits all wavelengths of visible light. However, incandescent bulbs do not emit the entire visible spectrum, and often, they emit different segments of the visible spectrum than fluorescent bulbs. The spectrum of solar energy that reaches the earth varies from mid-day to late afternoon, as the earth turns and the energy passes through more of the earth's atmosphere.
When you view objects that are made with different color pigments under different light sources emitting different visible wavelengths of energy, it's not surprising that those objects don't appear to be the same color.
How different can the same color appear?
Above is an example of how the same color paint, tinted with different colorants, appears as a different color under incandescent light than under daylight. Keep in mind the only difference between these colors is the light source used.
Understanding metamerism is important because it will affect the colors of various design materials if lighting conditions are not considered when selecting materials. The best way to avoid unpleasant surprises is to know the predominant light source the objects you're working with will ultimately be viewed. Then work under that light source when making selections or matching colors.