Ezekiel 23:14 speaks of the Babylonian army wearing vermilion, which is a shade of red. Nahum seems to be describing the approaching Babylonian army dressed in scarlet and with shields painted red. Red is a bold colour designed to make a marching army stand out and it also suggests spilt blood. The image is of a powerful, ruthless, and terrifying enemy.
3 The shield of his mighty men is red; his soldiers are clothed in scarlet. The chariots come with flashing metal on the day he musters them; the cypress spears are brandished.