The Haynes manual doesn't seem to distinguish the discoloring of the thread area, I have the same outer coloring as you. Perhaps some fuel just get stuck on that part and sits and burns to an black ooze there. But for the electrode stuff:
"Normal- Grey-brown deposits, lightly coated core nose. Plugs ideally suited to engine, and engine in good condition."
"Overheating- Electrodes have glazed appearance, core nose very white- few deposits. Fault- plug overheating. Check- plug value, ignition timing, fuel octane rating (too low) and fuel mixture (too weak)."
"Heavy Deposits- A build up of crusty deposits, light-grey sandy color in appearance. Fault- often caused by worn valve guides, excessive use of upper cylinder lubricant, or idling for long periods."
"Normal- Grey-brown deposits, lightly coated core nose. Plugs ideally suited to engine, and engine in good condition."
"Overheating- Electrodes have glazed appearance, core nose very white- few deposits. Fault- plug overheating. Check- plug value, ignition timing, fuel octane rating (too low) and fuel mixture (too weak)."
"Heavy Deposits- A build up of crusty deposits, light-grey sandy color in appearance. Fault- often caused by worn valve guides, excessive use of upper cylinder lubricant, or idling for long periods."
