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Index Color: The Fastest Proof-to-Print System?

(February 2008) posted on Wed Feb 06, 2008

Index separations are often avoided by those who fear being stuck with too many colors to print. This month, Trimingham debunks some myths about index color and explains how to use it as an effective tool in prepress.

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By Thomas Trimingham

The downside to this method of proofing is if the customer has a lot of changes, you will have to start over on the artwork and index it again. This is offset somewhat by the quickness with which the separations are done, but it is a concern if you have a nightmare client who pushes a lot of revisions through. The key is to practice with the dots and the file preparation so that everything works together to build a better proof and final print. The profit in proofing with index and then separating from the same file is first measured by speed and efficiency, then multiplied by ease of use on press with fewer issues concerning moiré and dot gain.


A Quick Rundown of Index Separation

1. Start with an RGB file that is actual size and the exact resolution of the dot size you can hold on screen.

2. Use the >Mode>Index Color dialog to bring up the index color dialog. Uncheck the preview button and select Custom from the Palette box.

3. The Index Color table will open. Click the lower right box and drag all the way up to the upper left box and let go. The color picker will open. Select white as a color. The box will open again. Select white again to clear the table as white.

4. Begin by creating the first upper left box as white and the next one in the top row as black.

5. Start by selecting the most saturated colors in the image. What you are looking for are colors that cannot be created by mixing other colors. Don’t forget to grab some grays or neutrals to create highlights and shadows.

6. After you feel you have all of the prominent colors in the design, select OK and then OK again. The design will change to index.

7. The image will probably not be perfect on the first color table. Zoom in and use Cmd or Ctrl Z to undo, and then redo the index and look at areas that seem to be harsh or rough transitions. You may want to add a color or change the table to correct these areas. You can just undo the whole index set, and when you >Open>Custom again, the last palette used will be there already as a starter. Just edit it or add to it until the result looks good. Remember to view the design at 100% or 50% to see it properly.

8. After you have the color table set, look at each color and see whether there is a way to combine it with similar colors. You can drop out the underbase to darken it or add a percent of another color into it. There are many ways to crunch the table down at this point, so be willing to experiment—but be realistic at the same time.

9. To create an underbase all you need to do is select all of the colors in the color table that need white underneath, change them to black, and change all of the other colors to white. Instant underbase!

10. For each separation color, just select that color and change it to black and all of the others to white. Copy and paste this image into a new document layer or channel and then undo it and continue with subsequent colors until complete.


Thomas Trimingham has more than 16 years of experience in screen printing as an award-winning artist, separator, industry consultant, speaker, and author of more than 40 articles in industry magazines. He can be reached through his Website,


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