The color of a cigars’ ash is determined by the magnesium content of the tobacco. The more white the ash, the more magnesium it contained and conversely, the darker the ash, the lower the magnesium content.
While other factors such as ageing can certainly come into play, cigars with higher magnesium content can be sweeter and smoother, whilst those with low magnesium content can taste somewhat bitter.
While this is the generally accepted rule, there are exceptions. Most notably, Cuban cigars, reptuted to be the Rolls Royce of cigars.
Where does the magnesium come from? The magnesium comes from the soil in which the tobacco was grown. It can also come from mineral rich fertilizers.