Spark Plug: Inspection: Overheating
WARNING: This page is about a different car, the 2003 Subaru Impreza. However, it is still accessible from the selected car via links, so may be relevant.
White or light gray insulator with black or gray brown spots and bluish burnt electrodes indicate engine overheating. Moreover, the appearance results from incorrect ignition timing, loose spark plugs, wrong selection of fuel, hotter range plug, etc. It is advisable to replace with plugs having colder heat range.