Printing Errors or Lights & Stuck Print Jobs

My officejet pro 6835 multifunction printer (HP) prints documents just fine,such as email or anything off of the computer. Ink levels are sufficient in the three colors and in the black. When I try copying a document and get the option of printing in black or color, I select "black" and the copy starts off being printed in black, but then the black starts to "fade out" and becomes red instead. There are no error messages, the "scan doctor" reports everything is working fine, there are no recent software nor driver changes, and our internet connection is wifi (wireless). It makes me think that the computer is not using black ink for copying, but raher "making" black ink from a combination of the colors and midway one or more of the colors "turn off" and the predominant color is left to finish the job. But it  should be copying by using only the black ink cartridge. Should be. What am I doing wrong - how do I fix this now out  of warranty printer?