This is an interesting issue. If the magenta is missing from most prints but not all that indicates, as you said, that this issue is something related to the software or the firmware. So currently I am going to have you do three things :
1. Pring a Print Quality page - this should be found in the Tools menu
2. I want you to reinstall the drivers after making sure the printer is removed from Devices and Printers, Programs and Features and the Device Manager. Use this link and not the CD to reinstall -
http://h10025.www1.hp.com/ewfrf/wc/softwareCategory?cc=us&lc=en&dlc=en&product=1153754
3. Following that same link make sure that there are no firmware updates or general updates that need to be applied
Respond back after you have done that and let me know what goes on!
-Spencer
PS I hope you are having a rad day!