In this part of the article, I’ll explain how to convert an RGB image to an indexed color image, how to choose the colors, how to create channels, and how to output them to the printer. In this example, we'll be creating a simple indexed color print.
Also see Perform Spot Color Separations
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.
To convert an image to indexed color, create spot color channels, and output the separations, follow these steps:
1. Open the file you want to use for your indexed color separation experiment. Verify the mode is RBG (Image>Mode) and that the design is the size it will be printed on the shirt. Make sure the resolution is 150-200 dpi (Image>Image Size).
2. Open the Info and Channels palettes. Zoom in on the image if necessary. You want to be able to see all of the colors clearly. [See Image 1]
3. From Image>Mode, choose Indexed Color. Move the Indexed Color dialog box out of the way so that you can clearly see the image.
4. Check Preserve Exact Colors, change Amount to 100%, and verify that Diffusion is selected.
5. In the Palette drop down list, choose Custom.
6. The Custom choice in the Palette drop down list opens the Color Table. Figure 2 shows what your screen might look like.
[See Image 2]
7. The colors in the Color Table have nothing to do with the image on the screen. Click at the top left corner of the Color Table and drag down to the bottom right to select all of the boxes, and from the Color Picker that comes up, choose white. You can choose white by clicking and dragging off of the left top edge of the Color Picker window. Click OK in the Color Picker dialog box. Click OK in that box again. The Color Table shown in Figure 2 should not contain any colors now.
8. Position the Color Table so that you can see the image. Click in the first box (in the top left corner of the Color Table window) to select it. The Color Picker window appears again.
9. Use the eyedropper to select the most common color in the image. Click OK in Color Picker.
10. Click the second box in the Color Table to select it. The Color Picker window appears again.
11. Use the eyedropper to select another common color in the image. Think about what colors can be used to create others too. For instance, if you know you plan to pick red and yellow, there's generally no need to also pick orange; Photoshop can create that color using the colors already selected. Don't forget to add black; it's almost always a necessary color. Click OK.
12. Click the third box in the Color Table to select it. Use the eyedropper to select another color in the image. Click OK. Continue in this manner until you've selected the colors you want. Pick at least 4, but no more than you have stations on your press. (Don't forget the underbase and highlight plates either!)
Here's an example of a Color Table with 8 colors selected.
[See Image 3]
13. When finished, make a note of each color you selected, save the color table using the Save button, and click OK in the Color Table and the Indexed Color windows. (You can use this saved color table later, if you need to edit your color selections.)
The design might not look so good at this point. That's okay; we've still got work to do.
With the indexing complete, you now have to create channels for each of the colors you chose in the indexing part (steps 1 through 13), so that you can tell Photoshop to print them out. For safety's sake, you might want to create another duplicate of the file at this point and save it under another name. Zoom in on the image, and then:
<Continued in Part III>