Why did Doc. inject Spot (carbon black) into large Polyp

I had my first colonoscopy today because it was recomended by my doctor because I am 50.  I had 2 polyps.  One was 3 mm and was completely removed.  The other was 20 mm and could not be completely removed.  It looked very big, with a lot of bumps on it and tiny red veins through it, to my untrained eye.  The doctor injected it with Spot (carbon black) for tattooning.  He used jumbo cold forceps but could not remove it completely.  I have an appointment with him in 2 weeks to talk about the biospy results.  My question is, Why did I get the carbon/tatto in the polyp and should I be worried?  FYI: I don't have blood in my stool, but I have been having lower back pain for a while for no apparent reason.  Thank you for any answers/openions.