The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 1. By Sybex | 5 | WebReference

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 1. By Sybex | 5

The Hidden Power of Photoshop CS: Chapter 2: Color Separations. Pt. 1

Adding Color to Tone

One thing that is simple when you hear it, but not so obvious, is that color depends on tone—not just as a means of encoding the RGB color, but to represent color intensity, depth, and brightness. You can’t have a dark color without dark tone, and you can’t have light color without light tone; tone and color go hand-in-hand.

We’ve taken a look at pulling tone out of color and also at how to add RGB separations back together to make a color image (in the first chapter). There are a number of ways to add color to tone, including making duotones, creating gradient mapping, and painting in custom color (or essentially what amounts to hand-coloring). Let’s look at these options next.

Applying Duotone to Color Black-and-White Images

A duotone is an image using at least two tones rather than just black (as in a simple grayscale image). When creating your own duotones, you replace the single tone of your grayscale image with two or more tones. The tones often work together to create a subtle, colored effect in an image, such as sepia toning. Originally, duotoning was a process used to introduce color and richness to black-and-white prints in the darkroom using a chemical process. In digital imaging and color process printing, duotones are used to achieve a toned effect (creating the colorized equivalent of a duotone color), to create a richer feel for black-and-white images, or to add a little color and increase the dynamic range of a printed image.

Figure 2.12 Setting the Hue to about 10% will give a decent sepia when you use the Colorize option with any black-and-white image.

Photoshop lets you create a duotone digitally in a number of ways. For example, you can rather quickly emulate a sepia tone by opening the Hue/Saturation dialog box, clicking the Colorize option, and shifting the slider to achieve a duotone effect (see Figure 2.12). When you print to an inkjet, the result will often be satisfactory.

This may give you the effect of duotone color in RGB, but it may not be the best solution for achieving the ultimate duotone effects in print. Certainly duotones can be pleasing to the eye, but the result would probably require all of the CMYK colors to print. This means that it wouldn’t technically be a two-toned (duotone) print.

In many cases, duotone images are used in spot-color jobs where there is a desire to limit color due to budget constraints but some color interest is needed. To create effective duotones for this purpose, you have to be able to control the separation for two inks (e.g., black and a spot color). If not, you will have to pay a technician to do the separation for you, or worse, pay for four-color work (CMYK printing) on what could be a two-color job—and then possibly not get the color you thought you applied (some spot colors will not separate to CMYK equivalents for printing). Using more than one ink to print your halftone images takes advantage of multiple screening angles, which can do a better job of presenting ink and color. Other advantages to using duotone over black ink alone include both lessening the appearance of halftone dots and increasing ink coverage. Creating a true duotone effect can also help with correcting and emphasizing subtle tonal detail.

Solving the problem and understanding the application requires a little knowledge of duotoning as well as knowing how to work with separations and spot color. We’ll take a look at the whole process, from breaking down and applying spot-color inks in preview to getting actual duotone print effects on your inkjet and on press—with and without using Photoshop’s Duotone function.

Background on Duotones

A key difference between duotone color and four-color printing (CMYK) is that four-color printing attempts to imitate color that exists in the image. Duotoning is pretty much the opposite: It is a means of adding color based on existing image tone. The image tone (either as grayscale, or however the color is converted to black-and-white) is used once for each ink/color. The color components are adjusted to influence which colors display more prominently to achieve specific shading and color effects.

The components are considered “ink” because the idea is to set up a representation of inks that will be applied in print in separate ink passes. The mix of the inks is controlled using tone adjustments (most often curves) to specify how the individual inks are emphasized in specific tone ranges. This adjustment will most often be different for each ink/component.

Selecting and mixing color using the tone adjustments controls how the colors interplay and affects the duotone result. The changes applied to the tone of each component manipulate how selected inks are applied. When the image goes to print, the adjustments create the interplay of color in the printed result. The component colors, or channels, become synonymous with the inks they represent on a press.

Manipulation of the image tone is usually controlled using curves—though it can be accomplished in other ways as well. Setting the adjustment (curves) without some sort of technique or method might prove quite fruitless and frustrating in that the results could be difficult to predict.

Likewise, picking color you like rather than color that will be effective can be troublesome in producing the desired result. Your plan for creating a successful duotone image should be:

1. Select inks that are compatible and that can accomplish your goal in duotoning.

2. Set up the image and apply curves to make the most of the inks you have chosen in the image you have.

There is a little art to experimenting with color selection and application, but some pretty straightforward science is involved as well. A 25 percent gray ink at 100 percent strength still shows at 25 percent gray when printed; it just covers 100 percent of the paper (see Figure 2.13). It can never get darker than 25 percent gray unless mixed with another color. With this in mind, any color affects an image mostly in tonal areas that are lighter than the 100 percent strength of the color. That is, a 25 percent gray ink will be able to more effectively influence tonality in 1–25 percent grays—though it will affect darker colors that it mixes with, it will be less effective in manipulating the image between 25 and 100 percent gray. A dark color can represent lighter tones, but the opposite can never happen. A good rule of thumb is to use at least one color that is dark or black for your duotone so that you can maintain the dynamic range in the image.

Figure 2.13 Magnification shows black ink (100 percent black ink) at 25 percent gray (25 percent coverage) and gray ink (25 percent black) at 25 percent gray (100 percent coverage) in halftone printing.

Figure 2.14 Unrealistic expectations can yield bad results in your duotones. You want blended tones to be
able to flow evenly from dark to light (A). Bad choices can limit tonal range (B) or lead to applying color inappropriately (C).

When choosing colors for your image, work with colors that are harmonious and sensible. In other words, select colors that can blend easily to produce a smooth white-to-black gradient (see Figure 2.14). For example, you might choose a black, red, and light yellow, or a black, blue, and sky blue to keep tones in the same family to make a successful tritone. The emphasis of the colors should be in their most effective range. Harmony between colors will make it easier to set up effective blends, and it will allow you to work successfully with various levels of image tone. If you choose difficult color combinations or have unrealistic expectations, you will have twice as much trouble getting the blends to be effective—or you may simply make it impossible.

Simply put, the lighter colors you use emphasize the contrast and tone in the brighter or highlight/midtone range of the image, and the darker colors emphasize the shadow details. You will most often want to use colors with varied tone, rather than all dark or all medium-toned inks. Splitting the effective range of the colors you pick can improve your image by giving you a choice of colors that have varying specialties. If you select a light and dark color for a duotone, the light color can be run with a greater density in the lighter image areas and the dark in the shadows. This will help you to most effectively work with (rather than against) image tone. The mixture of inks can give the image more of the feel of continuous tone and create a more photographic appearance.

Curves serve as translators for how ink density is to be applied. The curves are applied to the original tone of the image to target application strength. Throughout the tonal range, the inks blend to form shades and tones, which can be finessed to the advantage of the image. A good rule of thumb is that if you want to apply a 25 percent gray ink, curves should show high intensity at 25 percent on the curve graphing for that ink (shown in Figure 2.15).

Figure 2.15 This is an example of how a curve might look when using a light (25 percent gray) ink using both standard curves and the curves from the Duotone dialog box. Depending on the color, desired effect, and image, the curve can be even steeper than pictured here.

Deciding how to set the curves is part art and part science. Darker inks are often steeply graded in the shadow tones, whereas lighter inks are more steeply graded in lighter tones. This often easily and naturally makes a desirable result with lots of contrast. Other inks require different handling. If you use two shades of black ink, for example, you probably want to de-emphasize the total density of the inks or you will darken the image; however, in decreasing density, setting both curves the same way may not take advantage of potential dynamics in the image. Figure 2.16 shows curve settings used in a scenario with two dark inks.

When applying the same duotone colors to different images, you can’t always apply those colors with the same curves. The result depends on the image and its tonality. What works fine for normal and low-key (naturally dark) images may not work as well for high-key (naturally light) images, which would probably suffer from the attention to shadow detail. Curve presets might get you in the ballpark (you can save your curves using Save in the Curves dialog box), but you have to adjust the curves for optimal performance as carefully as if you were making image corrections.

You can use more than one technique to build a separation-ready duotone in Photoshop. Separation ready means that you will be able to use the duotone with process color printing. These techniques include creating a manual separation and using the Photoshop Duotone function. To fully understand the process, we’ll do the manual version first, and then we’ll use Photoshop’s function. Being familiar with both techniques will give you more control over the result, as well as a better understanding of using spot color.

Figure 2.16 In applying two black inks in a duotone, this example shows curves that emphasize contrast in the shadows (Black 6) and contrast in a smaller portion of the range (Black 7), almost like you would use a curve for tone correction.

Created: March 27 2003
Revised: March 1, 2004