How do I keep my blues from printing purple?
File Setup & Color
In CMYK, blue sits very close to purple. A blue made of 100% cyan and 100% magenta looks blue on most screens but prints noticeably purple.
Keep at least a 30% difference between cyan and magenta in your blues. For example, C100 M70 Y0 K0 prints as a strong blue. The rule of thumb: whenever you use a lot of cyan, use a lot less magenta.