The blue burnt part is hardcore chemically changed, so I think you'd have a tough time getting rid of that unless you cut it off or burned it more. I have the same cat back and I haven't noticed any goldness on my tip, but I haven't really looked at it super closely in a long time. You could probably rub it lightly with some metal polish and see if that does the trick. My guess is you have some sort of dirt on it.
07 WRX (Albins 5MT + Blouch 20G)
Jorge Carrillo Tuned (365 whp/340 wtq)