This may be the hardest subject for anyone to understand who does not routinely work with print. Traditional printing, which includes letterpress, generates color in two ways, process color and spot color.
Process Color Separation
“Full color” or “process color”, uses four inks (cyan, yellow, magenta, and black) to generate a large spectrum of color. It is a trick: halftones are created from the output files, their intensity depending on how much of each ink is needed at that point to generate the correct color mix. Furthermore, these halftones are set at specific angles, each one about 30 degrees from its neighbor, so that printing them on top of one another does not create moiré patterns. When printing color halftones, it is not necessary for the designer or prepress operator to manually separate the colors; RIPs do that automatically.
Note: it is possible to print separations from the print dialog box in both InDesign and Illustrator. On the Output tab, under Mode, select Separations (host-based) (AI) or Separations (ID). This will make the list of colors active and you will be able to select which ones get output and at what angle. For process color output, it is recommended that you leave the default angles set.
Spot Color Separation
The second method of printing colors makes use of individual inks, or spot colors. There are a number of different systems in existence, but the most commonly used in print shops is the Pantone Matching System (PMS). This consists of a series of base mixing colors which can either be used individually or combined to create thousands of other colors.
In order to maintain consistency, Pantone provides printed swatch books with samples of every color along with the proportions needed to mix it. Because ink behaves differently on different types of paper, they print books on uncoated, coated, and matte paper stocks. These correspond to the U, C, and M designations that appear after PMS colors in the software versions of the swatch books. Theoretically, the ink would be the same in each case, although in fact some of the colors are mixed differently depending on the type of paper being used. It is not necessary to include the U, C, or M when specifying PMS colors to printers; they will use the mix and match the swatch book with the correct paper type. However, you should be aware of which book to look at when selecting your colors so that you do not expect it to look very different from the actual output.
Note: Letterpress is primarily printed on uncoated paper, often with some texture that shows off the process to best advantage. Occasionally, however, printers get requests from designers to match a spot color to the coated book rather than the uncoated book. This is frustrating, partly because it represents a lack of understanding of the process on the part of the designer, but mostly because it is not possible except by creating a custom ink mix, and even that does not always work.
In order to output files correctly for print, art within them needs to be separated properly. This can be done using any swatch, provided it is a spot color swatch rather than a process color swatch. In practice, printers like to see the correct swatch that represents the ink they will use on press; this makes proofing easier and more satisfying and creates the correct labels on laser printouts that accompany the project to the pressroom.
The Pantone color books can be found in InDesign and Illustrator from the Swatches palette. In InDesign, this is straightforward: select “New color swatch” from the dropdown menu. When the dialog box appears, use the pull-down menu next to “Color Mode” to select, e.g., “Pantone + Solid Uncoated”. All spot colors are found either in the Solid, Pastels & Neons, or Metallics swatch books; CMYK and Color Bridge are for process color printing and should not be used for spot color output. In Illustrator, it is a little more complicated. In the Swatches palette, go to “Open Swatch Library” in the dropdown menu and follow it to “Color Books”; here you will see the same list of swatch books as in InDesign. A palette for the book you select will appear, and you can scroll through it or use the find window to type a number. You will need to click on it to add it to your swatch palette.
Spot swatches in Illustrator are shown with a little dot in the triangle at the lower right corner; you should make sure that any swatch you apply to art in your file has that dot, even if it is not strictly a Pantone swatch. You can easily change a swatch to be spot rather than process by double-clicking on it and changing the Color Type from process to spot.
Make sure that all art in your file has a spot color applied to it (strokes as well as fills). Do this by selecting the art and clicking on the correct swatch. As stated above, it is best to delete all of the swatches in the swatch palette that you are not using. By default, Illustrator shows dozens of swatches when you create a new document, and none of them are spot colors. InDesign does this too by default, but you are able to change the default so that new documents don’t have extra swatches. Thus far this is not possible in Illustrator.
The only color that it is standard to leave as a process color is black. However, this can create some pitfalls: your color settings may differ from your printer’s, so what may appear solid black (by which 100% K, 0% C, M, and Y is meant) on your computer may show up as a mix of the four process colors on your printer’s. Prepress operators should verify that black is really black, but never object to having the issue removed by receiving a file with a spot black instead. Whether you use a spot or process black, make sure that all black art is colored with the same swatch. This will make life more pleasant for the prepress operator, who will otherwise have to hunt through the file and update the color bit by bit. Illustrator does make this somewhat easier: under the Select menu, go to Same and then select the relevant search criterion, such as fill color or stroke color.
Don’t use Registration instead of black; it may look the same on screen, but Registration is a special swatch that will output onto every plate. If your file has fifteen spot colors in it, art that is colored with Registration will appear on all fifteen plates. This swatch is there primarily for printers to use when adding press marks to a layout file, so it is best to ignore it entirely.
Rich black is a process color solution designed to create a deeper, blacker black than standard black ink. This is done by printing 100% black with the addition of each of the other process colors, usually something like 30% magenta and yellow and 10% cyan. If you want to use this in a process color project, talk to your printer about how to prepare your file.
ICC profiles can be set and applied to layout files. When you are doing process color work with a color-calibrated printer, these can be really valuable. However, if you aren’t, there is very little reason to include them in your file, as they just increase file size and do nothing necessary. Spot color art does not use them at all. So when you are saving Illustrator files for letterpress output, uncheck “Embed ICC profiles” in the Illustrator Options dialog.
Trapping is what printers call the process of overlapping light spot colors into darker ones so that when printing minute changes in registration do not result in tiny white spaces where the colors do not quite line up exactly. You may notice that in InDesign and Illustrator, on the Output tab of the print dialog box, it is possible to turn on application-based trapping. In the vast majority of cases, this is not something designers need to worry about or do themselves. Different print processes, and even different presses using the same process, require different amounts of trap (if any). It is best to leave to the printer to decide whether trapping is needed and if so how to apply it.