The combustion of tobacco and the resultant chemical compounds formed is still not fully understood as yet. It can be anything, maybe a concentration of some trace elements in the leaf, a spot of irregular combustion, some introduced impurities during the processing of the leaf, etc. that led to the irregular colouring of the ash.
If you look closely you actually get all sorts of other colours: pink, red, blue etc. and even some colour changes as the unstable compounds formed react with oxygen in the air.
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