Dark rum gets its colour from oak ageing. The intense tropical heat expands and contracts the barrels and the wood becomes porous, like a sponge, forcing the spirit in and out of the oak. This adds all of the flavours present in wood – sweetness, vanillin, tannins etc. Rum is often aged in bourbon and sherry barrels and this in turn brings additional flavours - a raisiny, syrupy sweetness in the case of sherry ageing, or toasty vanilla and smoke from bourbon barrel maturation. Some dark rums are so dark that they could safely be called black rum...