I could write a book on this problem. My experience has mostly been cross contamination from one ink cartridge to another. Sometimes as ink levels change there is a suction from one cart. to another. Basically one color is sucked up into the damper of another. I usually try a medium clean and if it's only alittle contamination that will do it. Otherwise I print solid blocks of the contaminated color until it cleans itself out. You can also use a syringe to pull out the contaminated ink if you are knowledgeable about the printer and how to do it.
Happens to me alot for some reason. I find that the best thing is to keep these printers PRINTING!!. If they are working then they'll be no contamination
. I have trouble if mine sits for days or a week when it's a slow time.
Could be other problems like bad damper or captops or capping station as well.
Hope this helps.
John