Four-color printing is a method of color printing using the three primary colors (yellow, magenta, cyan) and black in the subtractive color process. It is a method of color printing using four colors: yellow (Y), magenta (M), cyan (C) and black (BK). If a printing process using other colors other than yellow, magenta, cyan and black inks to reproduce the original color is used, it should not be called "four-color printing" but "spot color printing"
Four-color printing is one of the types of printing. In theory, four-color printing can obtain thousands of colors, with a combination of four colors of Y, M, C and BK without repetition. Monochrome: Y, M, C and BK; two-color: Y+M, Y+C, Y+BK, M+C, M+BK, C+BK; three-color: Y+M+C, M+C+BK, Y+C+BK, Y+M+BK; four-color: Y+M+C+BK, a total of fifteen colors. In color printing, the size of the "dot" is used to indicate the depth of the color. In theory, the "dot" can be divided into 22 levels. According to the permutation and combination formula Cm·g, C1 4·22¹+C2 4·22²+C3 4·22³+C4 4·224=279840 (kinds) of colors can be obtained.
The so-called printing color sequence is the order in which the various color plates are overprinted in multi-color printing, with single colors as units. The new cyan, magenta, yellow, black (or black, cyan, magenta, yellow sequence) has become the commonly used color sequence.
Considering the brightness of the ink: the lower brightness is printed first, and the higher brightness is printed later, that is, the darker ink is printed first, and the lighter ink is printed later. Because the higher the brightness, the higher the reflectivity, the brighter the color reflected, and the overprinting of light colors on dark colors, the slight overprinting is not very conspicuous; and if the dark color is overprinted on the light color, it will be exposed. In general, the relationship between the brightness of the ink is: Y>C>M>BK.
Considering the transparency of the ink: the transparency of the ink is related to the hiding power of the pigment in the ink. The so-called hiding power of the ink refers to the hiding ability of the covering layer ink to the bottom layer ink. If the hiding power is poor, the transparency of the ink is strong, and if the hiding power is strong, the transparency of the ink is poor. Generally speaking, the ink with poor hiding power or strong transparency should be printed later, so as not to cover the brilliance of the previous ink and facilitate color reproduction. The relationship between the strength and weakness of ink transparency: Y>M>C>BK.
Consider the drying speed of ink: the slow drying speed is printed first, and the fast drying speed is printed later. If the fast one is printed first, for a single-color machine, it is easy to vitrify due to wet pressing and drying, which is not conducive to fixation; for a multi-color machine, it is not only not conducive to the overprinting of the ink layer, but also easy to cause other disadvantages, such as back smearing, etc. The order of ink drying speed: yellow is 2 times faster than red, red is 1 time faster than cyan, and black is the slowest.
Consider the properties of paper: a. The surface strength of paper. The surface strength of paper refers to the bonding force between fibers and fibers, fibers and rubber, and fillers on the surface of paper. The greater the bonding force, the higher the surface strength. In printing, the degree of powdering and hair loss on the paper surface is often used to measure it. For paper with good surface strength, that is, strong bonding force, and not easy to lose powder or hair, we should print ink with high viscosity first. Printing with high viscosity in the first color is also conducive to overprinting. b. For paper with good whiteness, dark colors should be printed first and light colors later. c. For rough and loose paper, light colors should be printed first and dark colors later.
Considering the characteristics of the original itself: Generally speaking, the original can be divided into warm-toned originals and cold-toned originals. For originals with warm tones as the main color, black and cyan should be printed first, and magenta and yellow should be printed later; for originals with cold tones as the main color, magenta should be printed first, and black and cyan should be printed later. In this way, the level of the main color can be more prominent.
Considering mechanical properties: Due to the different models of offset printing machines, their overprinting methods and effects are also different. We know that the monochrome machine is a "wet-on-dry" overprinting and overprinting form, while the multi-color machine is a "wet-on-wet" and "wet-on-dry" overprinting and overprinting form, and their overprinting and overprinting effects are also different. Usually, the color sequence of a monochrome machine is: print yellow first, then print magenta, cyan and black in turn. The general printing color sequence of a two-color machine is: black → yellow → magenta → cyan; cyan → yellow → black → magenta; cyan → magenta → black → yellow, or black → cyan → magenta → yellow, etc., among which magenta and cyan can be swapped according to actual conditions. The printing color sequence of a four-color machine is usually: black → cyan → magenta → yellow, or black → magenta → cyan → yellow. The above influencing factors and principles are only the printing color sequence arrangement selected by considering a certain factor alone when some other factors remain unchanged in theory. However, in actual printing, multiple factors often work together, so it is necessary to consider multiple reasons, give priority to the dominant factor, and arrange the printing color sequence reasonably. It is necessary to combine paper, ink and machine performance for comprehensive consideration to fully guarantee the final quality of the printed product.
