|The RGB (CMY) Color Model
RGB and its subset CMY form the most basic and well-known color model. This model bears closest resemblance to how we perceive color. It also corresponds to the principles of additive and subtractive colors.
Additive colors are created by mixing spectral light in varying combinations. The most common examples of this are television screens and computer monitors, which produce colored pixels by firing red, green, and blue electron guns at phosphors on the television or monitor screen.
More precisely, additive color is produced by any combination of solid spectral colors that are optically mixed by being placed closely together, or by being presented in very rapid succession. Under these circumstances, two or more colors may be perceived as one color.
This can be illustrated by a technique used in the earliest experiments with additive colors: color wheels. These are disks whose surface is divided into areas of solid color. When attached to a motor and spun at high speed, the human eye cannot distinguish between the separate colors and sees them instead as a composite of the colors on the disk:
Subtractive colors are seen when pigments in an object absorb certain wavelengths of white light while reflecting the rest. We see examples of this all around us. Any colored object, whether natural or man-made, absorbs some wavelengths of light and reflects or transmits others; the wavelengths left in the reflected/transmitted light make up the color we see.
This is the nature of color print production and cyan, magenta, and yellow, as used in four-color process printing, are considered to be the subtractive primaries. The subtractive color model in printing operates not only with CMY(K), but also with spot colors, that is, pre-mixed inks.
Red, green, and blue are the primary stimuli for human color perception and are the primary additive colors. The relationship between the colors can be seen in this illustration:
The secondary colors of RGB, cyan, magenta, and yellow, are formed by the mixture of two of the primaries and the exclusion of the third. Red and green combine to make yellow, green and blue make cyan, blue and red make magenta.
The combination of red, green, and blue in full intensity makes white. White light is created when all colors of the EM spectrum converge in full intensity.
The importance of RGB as a color model is that it relates very closely to the way we perceive color with the r g b receptors in our retinas. RGB is the basic color model used in television or any other medium that projects the color. It is the basic color model on computers and is used for Web graphics, but it cannot be used for print production.
Cyan, magenta, and yellow correspond roughly to the primary colors in art production: red, blue, and yellow. In the illustration below, you can see the CMY counterpart to the RGB model shown above:
Just as the primary colors of CMY are the secondary colors of RGB, the primary colors of RGB are the secondary colors of CMY. But as the illustrations show, the colors created by the subtractive model of CMY don't look exactly like the colors created in the additive model of RGB. Particularly, CMY cannot reproduce the brightness of RGB colors. In addition, the CMY gamut is much smaller than the RGB gamut (see below).
The CMY model used in printing lays down overlapping layers of varying percentages of transparent cyan, magenta, and yellow inks. Light is transmitted through the inks and reflects off the surface below them (called the substrate). The percentages of CMY ink (which are applied as screens of halftone dots), subtract inverse percentages of RGB from the reflected light so that we see a particular color:
In the illustration above, a white substrate that reflects 100% of the light is printed with a 17% screen of magenta, a 100% screen of cyan, and an 87% screen of yellow. Magenta subtracts green wavelengths, cyan subtracts red wavelengths, and yellow subtracts blue wavelengths from the light. The reflected light, then, is made up of 0% of the red wavelengths, 44% of the green wavelengths, and 29% of the blue wavelengths. The resulting spectral reflectance/transmittance curve would look approximately like this:
When printed on paper, the screens of the three transparent inks are positioned in a controlled dot pattern called a rosette. To the naked eye, the appearance is of a continuous tone, but when examined closely, the dots become apparent:
Note that in the above illustration, the cyan screen at 100% prints as a solid layer; the 87% layer of yellow appears as green dots because in every case the yellow is overlaying the cyan, forming green. The magenta dots, at 17%, appear much darker because they are mostly overlaying both the cyan and yellow.
In theory, the combination of cyan, magenta, and yellow at 100%, create black (all light being absorbed). In practice, however, CMY usually cannot be used alone. Due to imperfections in the inks and other limitations of the process, full and equal absorption of the light isn't possible; thus a true black or true grays cannot be created by mixing the inks in equal proportions. The actual result of doing so results in a muddy brown color. In order to boost grays and shadows, and provide a genuine black, printers resort to adding black ink, indicated as K. Thus the practical application of the CMY color model is the four color CMYK process.
This process was created to print continuous tone color images like photographs. Unlike solid colors, the halftone dot for each screen in these images varies in size and continuity according to the image's tonal range. However, the images are still made up of superimposed screens of cyan, magenta, yellow, and black inks arranged in rosettes:
Finally, CMYK printing, though it is chiefly regarded as a model of subtractive colors, is also an additive model in a certain sense. The arrangement of cyan, magenta, yellow, and black dots appear to the human eye as colors because of an optical illusion: we can't distinguish the separate dots at normal viewing size so we perceive colors, which are an additive mixture of the varying amounts of the CMYK inks on any portion of the image surface.
One problem that needs also to be addressed in discussing RGB and CMY is the issue of gamut constraints. The representation of the whole range, or gamut, of human color perception is quite large. However, when we look at the RGB and CMY color modelswhich are essentially models of color productionwe see that the gamut of colors we can reproduce is far less than what we can actually see.
While not precise, the illustration below clearly shows this problem by superimposing representative RGB and CMY gamuts over the 1931 CIE Chromaticity Diagram (representing the whole gamut of human color perception):
Both models fall short of reproducing all the colors we can see. Furthermore, they differ to such an extent that there are many RGB colors that cannot be produced using CMY(K), and similarly, there are some CMY colors that cannot be produced using RGB.
The exact RGB or CMY gamut depends on other factors as well. Every RGB device, whether a display monitor, color printer, color scanner, etc., has it's own unique gamut. Although the print industry has set standards for color production (e.g., SWOPSpecifications for Web Offset Publications), variances in presses, inks, and paper, as well as differences in environmental conditions within any given print house, affect the gamut of CMY(K) output.
These differences in gamut can create problems in the color production of computer-generated graphics and pages and inconsistent color is a problem inherent in all computer-generated color output.