Overprinting Colours
Overprinting one colour on top of another is a traditional way of mixing two or more colours. Software such as Adobe InDesign, QuarkXPress and Adobe Illustrator offer easy ways to turn on colour overprinting. However, overprinting often doesn’t result in the effect you’d expect. For example, what colour do you get if you overprint a 20 percent cyan box on top of a 50 percent cyan box? Most people assume the result would be 70 percent cyan. Not so.
The rules of overprinting say that no colour can overprint itself, and whatever colour is on “top” wins. So while overprinting 50 percent cyan on 30 percent yellow would work as expected (the overlapping area gets darker), overprinting 50 percent cyan on a mixture of 30 percent yellow and 75 percent cyan would not — the result is lighter than the underlying colour.
Fortunately, the Adobe Creative Suite applications let you preview your overprinting commands to make sure they’re going to work. In InDesign and Illustrator, choose View>Overprint Preview. In Acrobat Professional, choose Advanced>Overprint Preview. In other applications — such as QuarkXPress — consider exporting the file as a PDF and viewing the result in Acrobat.
Printing Overprinting
When you print a page that has overprinted colours, you still may not get what you expect. For example, some printers simply don’t understand overprinting commands, so when you print to them the effect is completely lost. The solution? In InDesign or Illustrator, turn on Simulate Overprint in the Print dialogue box or the Export PDF dialogue box.
In QuarkXPress, try exporting your document as a PDF file (using File>Export) and opening the PDF in Acrobat. You can print a PDF with overprinting commands to one of these types of printers by clicking the Advanced button in Acrobat’s Print dialogue box, then turning on the Simulate Overprint checkbox.
Note that you should not turn on Simulate Overprint for final, high-resolution artwork that is intended for colour separations. Simulating the overprint is only for a rough proof of what the colours will look like.
Use Multiply Instead
In many cases, it’s simply easier and more reliable to use the Multiply transparency blending mode instead of overprinting two colours. For example, in InDesign or Illustrator, choose the top object, open the Transparency palette, and choose Multiply from the Blending Mode popup menu. This always results in a darker colour where the objects intersect.
