Black and oily means the rings or valves guide seals are allowing too much oil into the combustion chamber. Black and dusty could mean its running rich. Brown and chalky probably means the engine is middle aged but ok.Cream,beige, off white is good.Bright white means its running too lean and may be pitted or corroded as well and can be caused by a vacuum leak!!!or overheating.If you put octane booster in they go bright orange from the sulpher dioxide.You havent got em all on the one motor have you?lol
i'v got 2 brown and chalky, 1 black and dusty, and one of the brown plugs has the heat looking discolouration on it on the same 2F. i haven't pulled the last 3 cause there was a heap of lines and stuff in the way and it was raining, and i was just passing time by pulling them anyway. a new set of plugs is on the cards now.
it runs LPG most the time if that means anything different the black definately looks like a carbon buildup