http://vector.tutsplus.com/tutorials/designing/quick-tip-how-to-create-mixed-ink-swatches-with-indesign/

Today’s financial situation has led to more clients asking for better value for money when setting budgets for design and print. A common request for a designer is to cut the number of inks from four down to two. Although this does cut print costs, it also cuts the designer’s color palette down dramatically. This tut will give you the knowledge to create more colors from a two color brief.