Color Printing

a method of reproducing multicolored images of paintings, color photographs, and the like on paper, cloth, or other materials. It is achieved by means of special plates, usually equal in number to the number of printing inks used. Normally, three are used: yellow, blue, and purple (three-color printing); however, frequently the three are insufficient to print dark colors, and it is necessary to resort to a fourth, which may be black or gray. Sometimes a fourth color is used to print a particular hue, such as turquoise or lilac. In such cases the process is known as four-color printing. In offset, which involves thin ink layers in impressions, the saturation of the prints is enhanced by introducing extra colors, such as dark blue and red.

In order to prepare each plate, color-separated negatives must be obtained. Color separation is achieved by photographing the original through color filters (transparent colored films or glass) that pass only the correspondingly colored rays reflected from the original. The filters usually used are blue-violet, red, and green, which pass, respectively, light blue, purple, and yellow rays. The original is photographed through a screen in order to print gradations of color. A multicolored print is obtained by the successive transfer of the appropriate colors from the plates to a sheet of paper, with the outlines of the images accurately superposed (in register). The eye perceives a single composite color, or hue, depending on which of the inks predominates in a given part of the image. The printing may be done on a multicolor press in one stage or on a single-color press in several stages.

Color distortions in color-separated negatives are removed in various ways, depending on the type of printing and the purpose of the print. The correction may be done directly on the color-separated printing plates by etching (for letterpress printing) or on the negatives and diapositives by a manual method, a photomechanical method, or an automatic method using electronic apparatus (seeREPRODUCTION PROCESSES). Color separation and color correction may also be performed simultaneously by automated systems, some of which provide finished color-separated printing plates directly from an original without the photographing and copying processes.

Apart from the selection of inks having suitable optical properties, the attainment of bright, saturated, multicolored prints is ensured by the use of very smooth grades of paper that have superior gloss and whiteness. Paper used for color printing is acclimatized to prevent appreciable linear distortion of the sheets, thereby facilitating proper registration of the separate images.