Brown ring around the base of the Porcelain(sp) is normal caused by high voltage micro frying oil particles, mist in the air attracted to the plug, nothing to worry about.
It is called CORONA DISCHARGE.
Have a close look for STREAKS up the porcelain as this indicated bad leaks of electric, normal for some bosch plugs as they are crap, and the streaking is tracking, will be all around the base of the plug .
tabetha