Matter of personal opinion but I find the Cuban ash absent minerals etc being the reason for gray ash false. Honestly I believe non Cuban ash is whiter because the tobacco is "cooked", whereas Cuban tobacco is not. Artificial heating sources are used to accelerate fermentation. This more extreem fermentation process removes more impurities making the tobacco burn more complete (to a white ash).

If you have a Weber filled with charcoal and a portion of the coals burn to white ash, but some portions remain darker, perhaps even black... do you figure the white ash burned more completely or do you believe the brickets that burned all the way to white ash had a higher magnesium count?