In printing, colors are created by mixing three primary-colored printing inks, cyan, magenta, and yellow (CMY). This method is referred to as subtractive color mixing because the ink filters the white light that falls on its surface, “subtracting” or absorbing all the colors of the spectrum except for the tone it was mixed to reflect.
Primary, secondary, and TerTiary colors
The color of the reflected light is determined by what wavelengths of white light are absorbed or reflected by the ink. Cyan, magenta, and yellow each allow in two-thirds of the spectrum of white light and filter out one-third. These ink colors have been chosen because the third that is filtered out is composed of some of the three colors the different cones of the eye are sensitive to: red, green, and blue. A magenta surface, for example, reflects the red and blue components of white light, while the green is absorbed. Red and blue light together become magenta, and that is why the surface appears magenta to us. In the same man- ner, yellow and cyan each filter out one-third: yellow lets in green and red but filters out blue, and cyan reflects green and blue but filters out red. CMY are called secondary colors to RGB.
In practice, black ink is used to complement the other three colors, from which comes the abbreviation CMYK and the concept of four-color printing. We will discuss this later in the chapter.
An unprinted surface reflects its own color–white if the surface is a white piece of paper, for example. In theory, mixing an equal amount of cyan, magen- ta, and yellow ink should create black, with the inks absorbing all visible wave- lengths of light, and the light being transformed into heat. A practical example of the transformation of light to heat is when you wear a black T-shirt on a warm summer day. The sunlight hitting the T-shirt is transformed into heat, and you will want to quickly change to a white T-shirt to reflect most of the sunlight.
In the CMYK system the resultant colors are defined by the percentages of CMYK inks in the combination. For example, a warm red color might be C = 0%, M = 100%, Y = 100%, K = 0%. If you don’t print with any color at all, you willet white, or the color that the paper or other material you are printing on has. The amount of ink used can vary from 0 (no ink) to 100 percent (complete ink coverage); coverage is accomplished with the aid of screen frequency values . We don’t recommend selecting colors based on what you see on your moni- tor, because it’s too difficult to achieve color consistency between the monitor and the final print. Instead, use color guides with examples of printed surfaces in predefined colors.
General color guides can be purchased on a variety of paper stocks, but printing houses sometimes supply their own color guides. Color guides define colors in CMYK values. If you are selecting a four-color hue, you can use a general color guide, but because the printing conditions vary (paper, printing press, inks, etc.) the result may not always be an exact match. If you want to be absolutely certain about the color, you should try to work with a color guide specific to the printing house that is printing your product.
Although mixing an equal amount of cyan, magenta, and yellow should theoretically produce black, the eye actually perceives a dark brownish color, especially on uncoated paper. This is because in practice ink colors are never completely pure and because inks do not adhere to each other perfectly. This is one of the reasons the three base colors, C, M, and Y, have been complemented with black in printing.
Another reason is that inks do not perfectly filter their third of the color spectrum, and because of this a neutral gray tone is not achieved when equal amounts of C, M, and Y are printed on top of each other. When 50 percent of each color is used, a reddish brown shade is the result. To compensate for this, a lower value of magenta and yellow is chosen to get a neutral gray: C = 50%, M = 40%, and Y = 40%.
One more reason to use black ink is because most printed products contain text that is to be printed in black. It is easier to print text with one color than to base it on a mixture of CMY, which would require exact control of the printing stages to make sure the text is legible.
As with the RGB system, the CMYK system says nothing about how the eye perceives the colors. A specific CMYK combination can vary in appearance depending on the ink, paper stock, and printing press used. The CMYK color gamut is much smaller than the RGB color gamut, which means that great care must be taken in converting from RGB to CMYK, using four-color separation for optimal color output.
The eye can also fool us. A particular color can be perceived in different ways depending on the color it is placed next to. A single color may be perceived as two totally different colors when placed next to different shades. This phenomenon is called the contrast effect. There is also the situation in which two colors that look identical in a particular light become completely different in another light. For example, a surface perceived as red when lit by white light may be perceived as orange when lit by yellow light.
For this reason it is important to standardize light relationships when you judge color in photographs, printouts, and printed materials. The color of the light is usually given in a color temperature measured in Kelvin (K). Neutral lighting has the color temperature 5,000 Kelvin. This corresponds to a mean value that natural daylight usually has and is therefore used as a referencelight for viewing images, printer’s proofs, and printed material. A higher color temperature yields a bluer light, while a lower temperature yields a more yel- lowish light.
There are many solutions for creating neutral light for viewing transparent images (slides and negatives), printed photos (copies on paper), printer’s proofs, and printed materials. For example, light tables or desk lamps can provide light with the right color temperature.