I. What Is Four-Color Printing
Four-color printing uses the three subtractive primary colors (yellow, magenta, and cyan) plus black for printing. If a printing process uses inks other than yellow, magenta, cyan, and black to reproduce the colors of an original, it should not be called "four-color printing" but rather "spot-color printing" or "point-color printing." Four-color printing is a lithographic printing method that achieves full-color reproduction through the subtractive color mixing principle using the three primary colors red, green, and blue, along with black colorants (inks or dyes).
Four-color printing is one of the types of printing.
It is a method of color printing that uses four colors: yellow (Y), magenta (M), cyan (C), and black (BK).
In theory, four-color printing can produce tens of thousands of colors by combining Y, M, C, and BK in non-repeating combinations.
Single colors — Y, M, C, and BK: four combinations;
Two-color combinations — Y+M, Y+C, Y+BK, M+C, M+BK, C+BK: six combinations;
Three-color combinations — Y+M+C, M+C+BK, Y+C+BK, Y+M+BK: four combinations;
Four-color combination — Y+M+C+BK: one combination, totaling fifteen colors.
II. Color Theory: How Many Colors Can Four Colors Produce
In color printing, the size of "halftone dots" is used to represent the depth of color. In theory, "halftone dots" can be divided into 22 levels. According to the permutation and combination formula Cm·g, the number of colors obtainable is C14·221 + C24·222 + C34·223 + C44·224 = 279,840 colors.
In practice, due to dot deformation errors during the printing process and the limitations of visual perception thresholds, the number of colors achievable in four-color printing is far fewer than the theoretical number.
III. Four-Color Printing Process and the CMYK System
The four-color printing process involves using four-color screen angles and four-color inks (three subtractive primary colors: cyan, magenta, and yellow, plus black) to reproduce the colors present in the original.
There are three main reasons why ink sequence arrangement is necessary: (1) the effect of ink overprinting and the shortcomings of ink pigments themselves; (2) paper quality; (3) the human eye's ability to distinguish colors. The most fundamental reason is the incomplete transparency of printing inks themselves — that is, the hiding power of the ink. The later-printed ink has a certain masking effect on the previously printed ink layer, causing the printed result to always lean toward displaying the latter color, or rather, leaning toward the mixed color of the latter and former colors.
1. Considering ink brightness: inks with lower brightness should be printed first, and inks with higher brightness printed later — that is, darker inks first, lighter inks later. This is because the higher the brightness, the higher the reflectance, and the more vivid the colors appear. Moreover, overprinting light colors on dark colors makes slight misregistration less noticeable; whereas overprinting dark colors on light colors would expose the misregistration completely. In general, the brightness relationship of inks is: Y>C>M>BK [3]. This can also be called process printing or CMYK printing — terms named after the printing inks.
Color images are printed using three subtractive primary colors (yellow, magenta, and cyan) plus black.
The color images used for printing must be color-separated into individual separation films, plus a black separation plate, to correct the cyan, magenta, and yellow inks and enhance the blackness of the printed image.
These films are used to make the four printing plates required for four-color printing.
On the press, each plate must be precisely registered with the other plates to achieve high-quality results. This is the industry-standard printing method.
CMYK is a color system widely used in computer printing devices [2].
IV. Limitations and Defects of Four-Color Printing
Due to the constraints of colorants and production processes, the C, M, Y, and K base inks used in four-color printing have spectral curves that deviate considerably from ideal color spectral curves.
The secondary colors produced by ink overprinting — CM, CY, and MY — especially blue and green, deviate even more from ideal colors.
With current ink production technology, there is limited room for substantially improving ink performance.
Therefore, it is difficult to significantly expand the color gamut of four-color printing.
For a long time, four-color printing has matched original density by measuring printed color density.
Measuring printed color density is essentially measuring the amount of printing ink used.
Printed color consists of chromatic components and neutral gray components. Under the condition of limited ink usage, if neutral gray components are excessive, it not only causes significant distortion in the brightness characteristics of the printed image but also relatively reduces the amount of chromatic components, resulting in decreased color saturation.
The visual lightness of four-color printed colors approximates a straight line, indicating that the neutral gray components of the printed colors are excessive.
Four-color printing emphasizes enhancing the saturation of reproduced colors at the cost of excessively sacrificing color brightness — this is another important defect of four-color printing technology.
Relative to the colors of natural scenes, due to the above defects, four-color printing reproduces colors that appear heavy, with considerable tonal distortion, unrealistic gradation, and weak three-dimensionality. The so-called printing color sequence refers to the order in which each color plate is overprinted, using a single color as a unit, in multi-color printing.
V. Printing Color Sequence: The Key to Color Reproduction
In traditional printing processes, the color sequence of yellow, magenta, cyan, and black was adopted because yellow ink was the most opaque at that time and had strong hiding power, making it necessary to print yellow first; otherwise, it would significantly affect the overprinted colors.
This traditional color sequence arrangement has gradually been phased out because modern yellow inks have relatively good transparency, reducing their impact on overprinted colors. This has also led printers to reconsider the traditional color sequence arrangement and use solid density as a means of controlling color, dot gain, and ink trapping on four-color presses. Consequently, the new sequence of cyan, magenta, yellow, black (or black, cyan, magenta, yellow) has become the commonly adopted color sequence [3].
VI. Principles for Selecting Color Sequence
6. Considering the characteristics of the original: Generally speaking, originals can be divided into warm-tone and cool-tone originals. For originals dominated by warm tones, black and cyan should be printed first, followed by magenta and yellow; for originals dominated by cool tones, magenta should be printed first, followed by black and cyan. This better highlights the gradation of the dominant tone.
7. Considering mechanical performance: Since offset press models differ, their trapping methods and effects also vary to some extent.
As we know, single-color presses use a "wet-on-dry" overprinting/trapping method, while multi-color presses use a combination of "wet-on-wet" and "wet-on-dry" overprinting/trapping methods, and their trapping effects are not identical.
Typically, the color sequence for single-color presses is: print yellow first, followed sequentially by magenta, cyan, and black plates.
Common color sequences for two-color presses include: black → yellow → magenta → cyan; cyan → yellow → black → magenta; cyan → magenta → black → yellow, or black → cyan → magenta → yellow, among which magenta and cyan can be interchanged depending on actual conditions.
The typical color sequence for four-color presses is: black → cyan → magenta → yellow, or black → magenta → cyan → yellow.
The above influencing factors and principles are only theoretical considerations for selecting a printing color sequence when examining a single factor while assuming other factors remain unchanged. However, in actual printing, multiple factors often act together, requiring comprehensive consideration of various aspects, prioritizing the dominant factors, and arranging the printing color sequence reasonably. This must be done in conjunction with paper, ink, and machine performance to fully ensure the final quality of the printed product [3].
