Electromagnetical radiation (more commonly called "light") is a mixture of radiation of different wavelengths and intensities. The light's spectrum records each wavelength's intensity. The spectrum of the incoming radiation is the physical variable underlying the color experience. As we will see, there are many more spectra than color sensations; in fact one may formally define a color to be the class of all those spectra which give rise to the same color sensation in humans.
A white surface reflects all wavelengths equally, while a black surface absorbs all wavelengths and does not reflect.
The familiar rainbow spectrum? - named from the Latin word for image by Isaac Newton in 1666 - contains all those colors that consist of visible light of a single wavelength only, the pure spectral or chromatic colors:
| red | ~ 625-740 nm |
| orange | ~ 590-625 nm |
| yellow | ~ 565-590 nm |
| green | ~ 520-565 nm |
| cyan | ~ 500-520 nm |
| blue | ~ 450-500 nm |
| indigo | ~ 430-450 nm |
| violet? | ~ 380-430 nm |
There are many colors which are not pure spectral colors, for instance brown? or pink?, or are iridescent or fluorescent.
The human eye contains three different types of color receptor cells, or cones. The first ("red") are most responsive to wavelengths around 565 nm, the second ("green") to those around 535 nm, and the third ("blue") to those around 445 nm. The sensitivity curves of the cones are roughly bell-shaped and overlap somewhat. The incoming signal spectrum is thus reduced by the eye to three values: the human color space is three-dimensional. If one or more types of a person's color-sensing cones isn't responding correctly to incoming light, that person is said to be [color blind]?, and has a smaller color space. Other animals may have more than three different color receptors (some birds and reptiles) or fewer (most mammals).
Because of the overlap between the sensitivity ranges, not all stimuli are actually possible. So, for instance, if green light enters the eye, all three receptors will react to some extent, with the "green" receptors showing the strongest response, while light at 565 nm will actually look yellow. If we plot the spectral colors on a chart showing the corresponding stimuli of the receptors, we find they define a curve, and that only colors inside the curve can be observed. Also, given lights corresponding to two or more points, we can duplicate all the points (up to human perception) in the region of the chart they bound by mixing them. This is called additive color mixing, and the points are called primary colors. No finite set of primaries can duplicate all possible color sensations, but an apropriate choice of three can cover most. For this reason, color television sets and computer monitors generate light by mixing red, green, and blue light together. The description of a color in terms of these primaries defines what is called the RGB color space; unfortunately there is no exact concensus as to what frequency the red, green, and blue lights should be, so the same RGB values can give rise to slightly different colors on different screens.
When producing a color print or painting a surface, the applied paint changes the surface in such a way that, when illuminated with white light (which consists of equal intensities of all visible wavelengths), the reflected light will have a spectrum corresponding to the desired color.
It is possible to achieve a large range of colors seen by humans by combining cyan, yellow and magenta? paints on a white surface ("subtractive light mixture"). The cyan paint will reflect all but the red light, the yellow paint will reflect all but the blue light and the magenta paint will reflect all but the green light. This is because cyan light is an equal mixture of green and blue, yellow is an equal mixture of red and green, and magenta light is an equal mixture of red and blue. To describe colors by subtractive mixture, the CMYK? (Cyan, Magenta, Yellow, blacK) color space is used.
The RGB and CMYK color spaces are most useful for technical reproduction of colors. A color space that more closely models the human experience is the HSV? color space which arranges colors in a three dimensional cone. If the pure spectral colors are extended by mixtures of red and blue, they can be arranged in a circle (which was already known to Newton), the mouth of the cone. The position of a color on this circle is its hue. In the HSV space, every color is specified by its hue, saturation and luminocity.
The color cyan exemplifies what was said earlier: the same color experience can be generated by different light spectra. Cyan is a pure spectral color, but it can also be generated by an equal mixture of the spectral colors green and blue. The human eye (as opposed to the bird's eye or the spectroscopist) can't tell the difference.
Different colors are often associated with different emotional states, values or groups. These associations can vary among cultures and will be explained on the pages describing the individual colors.
References:
Color, in journalism, refers to vivid, but peripheral commentary on an event, especially in broadcast sports.
Color, in law, refers to certain prima facie rights.
Color, in a deck of playing cards, refers to the four suits hearts, spades, diamonds and clubs.
Color, in politics, is associated with different parties or ideological factions. The classifications vary in different parts of the world, but red is generally associated with socialism or communism, blue with conservative parties and green with environmentalists.
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