Hi guys:
For years I have noticed this brown stain on my high voltage, large gap spark plugs. It worried me, and eventually discovered it is called a “corona stain”. See this link.
http://www.ngk-sparkplugs.jp/english/techinfo/qa/q17/index.html
If this link goofs up, just google “spark plug corna stain” and you will get it.
I believe this is correct, because oxidized oil readily dissolves in acetone, and acetone wipes this stain right off.
I think that the long plug service combines with a very high electric field to oxidize oil vapor in the plug well, and then electrostatically deposit this “fried” oil onto the plug porcelain.
See if your rust colored deposited wipes of with acetone, methanol, or nail polish remover (aka methylethylketone).
Sorry about all the technical vocabulary.