Verify what color profile you have applied under your Preferences > Display > Monitor Profile. On my test machine, running QuarkXPress 7.5 on OSX 10.5.6, a 10% cyan is clearly visible, and displays equally in Quark, in a PDF, or in any other imaging application.
At 5%, the tint is practically invisible, but that seems to me a limitation of today's electronic displays and/or our human perception.
Manuel S.
Technical Support Representative