Using Spot Colors in Channels and Duotones
Color Models, Spot Colors & Duotones
Each industry, Printing, Web and Video, and Designers have specific models to describe color. Although this class is mainly interested in the Printing industry, it is important to know how each model works. This will give you flexibility and broaden your ability to do different jobs, and communicate across fields. Each color model has is own range of colors called gamut or "color space". This lesson will focus on Print.
Models, using reflective light, generally have a narrower range or
gamut than Additive Models such as RGB which use projected light. Subtractive Color Models would include CMYK which is used for ink. The brightest white available is the white of the substrate (such as paper), and the more ink you add, the darker the paper gets.
It is important to choose the right color model for the job. If your images will be commercially printed, then ensure that they are in CMYK mode, and assure all colors used are within gamut (in printable colors) before printing. If your images are to be displayed on a computer, then make sure you use the RGB mode so the full gamut can be displayed. When talking with designers, you may need to refer to specific color charts such as Pantone®. Other color models include Grayscale, HSL, HSB, Lab, and XYZ.
The most common color model in the Printing Industry is CMYK. CMYK refers to the inks used by printers in typical four color process printing: Cyan, Magenta, Yellow and Black. CMYK is a "subtractive" color model based on reflected light. This means the more color used, the darker the image becomes. In this model, white is the absence of color. Tonality is achieved by using a half-tone screen to create dots of color. The larger the dots, the stronger the color. As these screened colors overlap, they optically mix and produce the illusion of full-color. To make a mixed color, Red for instance, you would have both magenta and yellow overlap. If you wanted to darken a color, you can add other colors or black. Red, when mixed this way would be called "Process Red." Any color within the CMYK gamut (range of colors achievable using CMYK on white paper) is a Process Color.
CMYK is fairly consistent, but variations in exposure can cause variations in colors. For jobs that need exact color matches, the Printing industry has recognized certain color matching systems, such as Pantone®. These colors are called "Spot Colors." These colors will give you an exact match no matter what printer you use anywhere (at least, that's the theory...). Normally, Spot color inks are used at 100% to take full advantage of their specific hue. However they can be screened or mixed with another color, such as black also. This is commonly done with duotones, but we'll talk more on that later.
Spot colors are special premixed inks used instead of, or in addition to, the process color (CMYK) inks. Each spot color requires its own plate on the press. (Because a varnish requires a separate plate, it is considered a spot color
In this example, from Adobe's Photoshop CS Classroom in a Book, we have an example from Lesson 13 on creating an image which will be printed in Black ink and a Pantone 124 C Spot Color Ink, found in the custom color pallet of the Swatches dialog box. The Black and Spot Color are on two separate channels. Each channel will be burned to separate plates and printed resulting in a two-color Image as seen at the right. The black and white screened separations look like the images below
Using the same image, we have converted it to a duotone. Duotones, are grayscale images printed with two inks. This technique produces an image with a slight tint to it and significantly increases the image's dynamic range. Duotones are ideal for two-color print jobs with a spot color used for accent. We have used the same Spot Color, Pantone 124 C, but we don't need to create a separate channel as we did before. The result is the tinted image on the right.
Duotones are used to increase the tonal range of a grayscale image and also to add the impact of color. Although a grayscale reproduction can display up to 256 levels of gray, a printing press can reproduce only about 50 levels of gray per ink. This means that a grayscale image printed with only black ink can look significantly coarser than the same image printed with two, three, or four inks, each individual ink reproducing up to 50 levels of gray.
Modifying the duotone curve
In a duotone image, each ink has a separate curve that specifies how the color is distributed across the shadows and highlights. This curve maps each grayscale value in the original image to a specific ink percentage. This can give you great sensitivity when creating your duotone. Below we have created a much richer tone by lowering the black curve, and raising the Spot Color curve.
Duotone from Scratch
For our assignment, we will not be using Photoshop's Duotone options. We will be working directly with channels. Channels hold the color information for your image. In RGB, you would have three channels: Red, Green, & Blue. In CMYK, you would have four channels: Cyan, Magenta, Yellow, & Black. You will be starting with a Grayscale image which only has one channel. You will then add a Spot Color Channel.
For your assignment, you will create a 8.5 x 11" Duotone Poster.
This will require a grayscale channel and a Spot channel. If printed these posters would only require two plates and two inks.
Adjust the gray scale and Spot color images using curves, paint brushes, dodge & burn tools or any other technique we have learned to enhance your image.
Add a title which will show up as white or at full Spot color.
Save as a Photoshop file! A jpeg does not know what a spot color channel is and you will lose it.