3rd spark plug looked different. Why?

It had been just over 3 yrs & almost 28k miles since I’d replaced the 4 plugs in my 1997 Geo Prizm (Toy.Corolla twin), so I did it yesterday, following my Haynes manual. All 4 electrodes were grayish–apparently normal.

But the 3rd plug (counting from the right)

surprised me: When I yanked out (no easy feat) the 5-inch plastic rod, there was a yellowish scarring toward the bottom end. I was able to rub off that residue with a paper towel.

What was that yellowish stuff? And why were the threads on that 3rd plug blacker than the others?

The Five inch spark plug wire boot that snaps on top of the spark plug had yellowish scaring toward the bottom that could be wiped off? Sounds like the spark plug was not completely seated on the head causing combustion gases to leak past the plug. Make sure new plugs are properly torqued. If it were me, I would also replace the plug wire.