Oreo Cookie Like Spark Plug

So I pulled my plug out and it is black on the outside and white on the inside. There is a fair bit of carbon in the exhaust pipe too but the bike seems to be running great. I ride at about 4,000 ft so have the JD red needle in 4th clip from top, 185 main, 45 pilot and fuel screw 1.5 turn out. It seems like rich jetting but it runs the best it ever has on the bottom end and I have nothing to complain about for top end performance and running either. I might try a 180 main next time I'm at the track though just for an experiment. So back to the plug, what does a white electrode but black on the outside and in the exhaust suggest?

Oh, the bike is an '08 TC250 with 45 hrs on it now. Just checked the valves today too, all still in spec from new, never had to shim yet. I have some pics but I'll have to figure out how to post later, need to cook dinner now!

Maybe dunk it in milk?

White on the inside tells me its a bit lean.

"it runs the best it ever has on the bottom end and I have nothing to complain about for top end performance and running either"- could also be a symptom of slightly lean, kinda like a carbed snowmobile runs better in the colder weather, because its a bit leaner.