Tab, if you read it pages 14,15,&16, dave quotes that plug colour has nothing to do with mixture condition, what you are in fact is looking at temp of the plug, as indicated by the colour of the carbon, if you fit a hotter running plug the colour of the carbon will be a lighter shade, this does not indicate a weak mixture, just a hotter running plug,
he goes onto say tuning by plug colour, the colour indicates plug temperature Not Mixture Ratio. and if you fit a colder running plug the carbon will be darker even sooty in appearance, its not rich just colder running, i think most people get side tracked by plug description as in haynes manuals.
Although it does contradict in that say you had a vacuum leak, and the temp increased in the cylinder the plug would change colour and the mixture AFR also? .i think dave w is assuming the engine state of tune is all good.
Regards Dave R.