The leather dye-tub can be used to dye leather or studded leather armor.
Using normal dyes on this tub (double-clicking the dyes then targeting the special dye-tub) will bring up a menu listing the colors available to use on leather armor.
After selecting a color, the dye-tub will then have that hue.
Double-clicking the dye tub and targeting a piece of leather or studded leather armor will dye that clothing the color.