Transparency

Other operations may result in undesirable results when spot colors are involved. For example, the blending modes Difference and Exclusion will cause overlapping spot color areas to be converted to CMYK (or RGB if that's the document's working color mode). You may also have undesirable results if you create spot-color objects, apply blending modes to them in Illustrator, and then place the files into another application such as InDesign. If you convert spot colors to CMYK in InDesign by using the Ink Manager or by editing swatches, the output may not show the same color interactions as the original Illustrator file. Illustrator provides a warning as you save the file (Figure 10.20).

Figure 10.20. Spot-color objects using transparency may not fare well when other applications render the Illustrator file as CMYK. At least Illustrator warns you. Don't check the Don't Show Again optionit's good to keep this warning in mind.

Even if you don't intend to convert spot colors to CMYK, Illustrator's display can mislead you about the final outcome. The most you can have of any given single ink is 100 percentsolid ink coverage. For example, you can't fill an object with 200 percent cyan. But if you fill two objects with a solid ink, and then apply a blending mode of Multiply, Illustrator will darken the overlapping area (Figure 10.21). It's an unfortunate display error. If you turn on Overprint Preview (View > Overprint Preview), the display is correct. Overprint Preview is a great forensic tool to help you diagnose numerous problems, but it does slow down performance. Use it when you need it, and then deactivate it.

Figure 10.21. The word and the rectangle are both filled with 100percent Pantone 415. The word uses the Multiply blend mode. The Illustrator display (top) falsely implies that there will be 200percent coverage of Pantone 415 in the overlap area, which is impossible without a second ink. However, if you turn on Overprint Preview (View > Overprint Preview), you'll see the true story (bottom).