Replace the Magenta imaging unit. I don't think you can swap drums. Order the drum imaging unit kit which comes with 4 drums.
Swap Magenta and Cyan drums, if the background turns more blue than pink, the drum is the problem (replace)
But take Quitte's post into consideration, sounds like experience talking there.
One of the Workcenter 6655i printers at my workplace has started acting up. Pages printed in black and white are totally fine, but if you try to print or copy in color, the page will be completely pink with after it's done printing (the image will also be there, but covered in pink). I'm making this post in the hopes that someone can help me fix it before I have to put in a call it in for repairs.