…indigo, the natural plant based kind. Interestingly, at the time of the Civil War, indigo was only grown in the south. It would merit research to study the trade and value during the War of dyes to make uniforms for both sides. We assume the gray of the South and blue of the North were achieved through the use of mordants.
(Photo: officer’s uniform from the War Between the States. As time has faded it, we can only assume this is the blue of the north. A good example of the duration of dyes based on use of mordants for staying power)