How can I keep blues from looking purple?

In CMYK printing, blue and purple sit close together in the color spectrum. To maintain a true blue, create enough separation between the Cyan and Magenta levels — generally at least a 30% difference.

For example, a formula such as C:100 M:70 Y:0 K:0 will produce a rich blue without shifting toward purple. Avoid using equal values of Cyan and Magenta, as this can result in a purple tone once printed.