Photoshop and Screen Printing - Perform Indexed Color Separations Part 1 of 2
written by: Joli Ballew•edited by: Daniel P. McGoldrick•updated: 2/24/2009
Indexed color offers options for screen printers that spot color doesn’t, and in Part 1 of this article you’ll learn the pros and cons of this technique and how to prepare your artwork.
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Whether you are new to screen printing or run an established screen printing shop, I'll bet you don't do a lot of indexed color printing. From what I can tell from my own experience, the majority of shops tend to go with standard spot and process color printing techniques. Indexed color offers things those can't though, and is well worth a look. In this article, I'll introduce indexed color, when to use it, and talk a little about the advantages and disadvantages of the process. Once I've convinced you to experiment, I'll show you step-by-step how to perform indexed color separations in Photoshop. Happy indexing!
Note: Some of the steps and screenshots used in this example are excerpts from my book "Photoshop 7.0 for Screen Printers", Wordware Publishing. You can purchase this book and its sequel, Photoshop CS3 for Screen Printers from my web site at www.joliballew.com.
What is Indexed Color?
Indexed color is a process where an image with several colors is reduced to a limited color palette, and where only a few, specific colors are used to create the image. You choose the colors yourself using Photoshop's Index Color dialog box and the Color Table palette. After indexing, the design is reproduced (or rebuilt, if you will) using the colors you chose in the Color Table. These colors are blended together (dithered) using a random pixel pattern made up of square dots that recreate the image as closely as possible. The square dots are much different than the halftone dot patterns created with process prints, and this pattern offers several advantages.
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When and Why to Use Indexed Color
Indexed color is a great technique to use when you can easily determine what the main colors in an image are. When choosing an image to use for this type of print, you have to decide if it is one that can be recreated realistically using 4-10 colors (or more, if you have sufficient stations at the press). The process can be used for images with a vast amount of colors as well, but I'd suggest working up to that after you've had a little practice. You can use indexed color for printing on both light and dark shirts, making it extremely versatile. Because you can print with opaque inks, the prints are bright and robust, and because of the shape and random nature of the pixels, there are rarely problems with moiré.
The dots, which are square, are all the same size and placed randomly as needed in the image, making printing indexed color much easier than printing process color. You can choose between a variety of screen types and mesh counts, and you can use all purpose inks. These things make this type of print a good option for small shops who are moving from spot color prints to other processes, and are a good transition from spot color to more complicated process printing techniques.
In my opinion, indexed color prints work best for images that are not photo-realistic, because the print tends to lose its realistic quality due to the lack of colors used for creating it. However, that's not to say that some really awesome, award-winning, photo-realistic prints haven't been created; they certainly have. It just takes practice and you have to have an eye for color. I prefer to use indexed color for spot color work that has mild to intermediate gradients, and for images that have a distinct number of selectable colors.
I've only see a few problems with creating indexed color images. One is the problem that arises when you try to take an image with lots of colors and transform it into one that only consists of 4. In this case, it's almost impossible to get an image that closely resembles the one you started with. If you have lots of color or wish to closely match the original image, you'll have to go with 6 colors or more. That makes a 6 or 8 station press a necessity, and if you don't have one, you're probably out of luck. Another problem arises when the image is geometric or has sharp, hard outlines. Because the pixels are square, it can make these edges appear less sharp and even fuzzy.
Despite the disadvantages, indexed color separations offer a neat way to move forward and experiment with additional techniques. Go ahead and give it a whirl; step-by-step directions are next.
Tip: Indexed color prints look better the more colors you use. If you can, create the print with 6-10 colors.
In order to create and use indexed color separations, the colors in the image itself need to be selected, indexed, and then separated into channels. Before you start the process though, you'll want to take care of a few things first.
Take a look at the colors in the image with your best artist's eye. Try to figure out what colors you think should be chosen in the color palette. Remember, you'll most likely be choosing 4 - 10 colors, and those colors will be used to create all of the colors in the image, so your choices are important. Selecting the colors is really the heart of indexing, so expect to spend some time here; it'll take a little while to get an eye for it.
Make a duplicate of the original too, just to be on the safe side. Clean up the image if needed, and prepare the file as you would for any other image you'd print. You might also want to create an underbase and/or a highlight white plate. Make sure to convert them to Bitmap mode, select Diffusion Dither, and use the same resolution as you'll use in your indexed file.
Note: It's quite possible that the first few indexed color separations you do won't turn out right. Choosing too few colors, choosing the wrong colors, and limitations on the number of stations you have on the press all factor in.