Some types of octane booster will also discolor the plugs.
There was an '03 owner who's warranty claim was denied soley because his plugs were discolored. They stated he'd been using octane booster in the engine which resulted in the damage...
i work for a petroleum company called Royal Petroleum, the discoloration is common among 100+ octane due to the stabaizer they use to prevent pre-detonation of fuel from the compression in high compression projects car in most casses. other times it can be from simple cleaning additives and thinners. Dont worry about it unless you start to see that colored smoke out ur tail pipe, then ur either running too rich or you have a seal break; including blue or white smoke fore oil and water burning