[Rufton]

Your plug looks great.
Keep in mind plug color can change.
Plug can be black after rich idle but turn white again after lean cruise.
I'd pull one plug after next time it idles bad to make sure idle isn't rich.

If you over advance timing you can destroy engine in a heart beat. 6000 rpms is starting to get scary because you are stress testing engines quality and design. My engine is designed for 6500rpm but I'm not brave enough to test.
Check all in timing at about 2500-3000rpm to make sure it does not go above design; many street engines like all in timing of 32 to 38.