[Roadsters] Sparkplug Tan vs Gray color

[Tom PRATHER]

Hi Listers,
                 I have searched the net about reading sparkplug color and
jetting but can never find a definitive answer as to the differences about tan
color vs gray. It's been written usually as tan/gray is just right as my SU
U20 used to run a tan. My current Mikuni Flatslides are running a medium gray.
Would gray be richer than say tan? My plug base ring is showing a charcoal to
black color and not rubbing off and no soot. They say it's better to be
slightly on the rich side to ensure engine safety so where do I stand? Any
opinions? Thanks Tom