Odd spark plug colour

[John Larson]

I remember hearing, in a long-ago class, that reading plugs from an engine
run on unleaded fuel is difficult because they don't display the same colors
and characteristics we're used to.  I could, of course, be wrong.  If it IS
the case, then those charts are pretty much meaningless.  The additives in
fuels today are apt to give us odd colors, as are the base fuels themselves.
High sulfur base fuels (like Shell on the left coast of the USA)  give a
bright yellow tint to the insulators.  HTH, John