For a long time printers have used the primary colors of process cyan, process magenta, and process yellow, along with carbon black (represented by the letter K for Carbon) to print comic books. Before digital coloring the books were colored through a painful process involving acetate film which was used to make printing plates through a photographic process. The exact process is a bit difficult to explain, but it involved a series of codes called “color codes” which are no longer used. I have included a chart of these color codes in case you’d like to learn them or need to restore an old comic book from the printing plate alone digitally (such as I have done for some of the Marvel Masterworks series).
The chart requires a bit of explanation. Firstly, it is in tiff format in CMYK color so that the color is not tainted by the standard degradation of the jpg format and so that it will print more perfectly than an rgb file would. It must therefore be opened in a program that can handle tifs and the CMYK color mode such as Adobe Photoshop or the colors won’t be seen correctly. Most image viewers do not support cmyk format and will shift the colors to an imperfect rgb so that the colors seen do not appear as they should.
Secondly while one would think that the code “R” represents red and “B” represents blue remember that pure red and blue were not used in the printing process. The “R” actually stands for process magenta (a very cool red containing a large percentage of blue) and the “B” actually stands for process cyan (a very cool blue containing a large percentage of green). The Y does however represent the color yellow, but process yellow is cooler (contains more green) than standard yellows such as Cadmium Yellow. The letters “R” and “B” were used only for the sake of simplicity and now represent a misnomer. Refer to the key below the link to understand how to interpret the chart.
To get the chart right click on the link below and click “save as” to download it to your computer.
and here is a Photoshop .aco swatches palette with the codes
Key to the Codes
Y = Yellow
R = Magenta
B = Cyan
2 = 25% color coverage
3 = 50% color coverage
A letter alone represents 100% coverage
Y = 0,0,100,0 or simply 100% yellow or total possible yellow coverage.
Y2 = in cmyk code 0,0,25,0 or simply 25% of total possible yellow coverage.
Y3 R2 B = 100, 25, 50, 0 or simply 100% Cyan coverage combined with 25% Magenta coverage and 50% yellow coverage.
A fully saturated non-carbon black could be made from simply Y R B which would be 100,100,100,0 in cmyk code, but because paper has a limit to how much ink it can properly absorb without breaking down it is considered a better practice to use some carbon ink in any mix that is particularly dark. This is because K tones are more efficient in toning down the color and so less ink is required preventing the page from reaching full saturation as quickly. Also carbon ink is cheaper and therefore it is preferred by printers any time black is needed rather than a mixture of colors that would result in a black. Also you should know that Karbon Black is a bit transparent so pure Karbon Black is usually “underprinted” with a combination of colors which lay underneath the black to make the black more solid and rich.