I changed the plugs on my son’s 2009 Sonata 4-cyl today. One of the coils (second from passenger side) was discolored; it’s the right-hand one in the photo.
The plug from that cylinder had some brown discoloration on the insulator and on the metal body below the hex; it’s the RIGHT hand one in the photo.
(The left-hand plug in the photo is the second-worst discoloration of the four. The best one was almost completely clean.)
What caused that discoloration, and is it anything to worry about?