Back in September 2009 robertvt asked a question about this (HP Color LaserJet 2605dn color alignment issue). I have the same problem. The fix was posted by Jamac47 at:
http://www.printer-repair.guidesonline.info/hp-color-laserjet-2605-colours-out-of-alignment/
This page no longer seems to exist and I cannot find the printer-repair guidelines site. I have searched around but cannot now find details of what Jamac47 said to do.
Can anyone help please? Many thanks