Why is my color changing as the poster prints?

When printing a poster the color starts out as red and then at some point during the printing it changes to pink. The first time I thought it was a fluke so I tried printing it again and it did the same thing but at a different point in the printing process. I am using a HP DesignJet Z5200 44-in Photo Printer and have printed multiple items prior to this without a problem. Any suggestions on a solution?