The best rums are made under the strict supervision of an experienced distiller, from fresh raw materials. Some people swear pot-still rums are the best, but without the patience required to age these spirits they are not really special. Rum gains a slight yellow to brown color as it ages in oak barrels. But some rum is aged and then filtered to remove the color acquired during the aging process. And not all dark rums are aged.
Distillers commonly add caramel coloring to their spirits to adjust the color.
Sometimes this is just to compensate for the differences in the color of the rum from different barrels, at other times caramel is used to make the rum so dark that you can't see through the bottle. Rhum agricole is a spirit made in the French islands from fresh sugar cane juice. There are other specifications, but these are the ones most important to the consumer. Ahhh yes, rhum vieux. This is rhum agricole that has been aged in oak barrels of less than 650 liters capacity for at least three years.