Dark rum gets its color from oak aging. 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 flavors present in wood – sweetness, vanillin, tannins etc. Rum is often aged in bourbon and sherry barrels and this in turn brings additional flavors - a raisiny, syrupy sweetness in the case of sherry aging, or toasty vanilla and smoke from bourbon barrel maturation.