Once you have the Moonless Midnight dye, select it from the Items list -->Dyes. It will then pull up a list of clothes that you can dye, make sure you dye the clothes you are wearing (should be highlighted). I got the achievement after dyeing my hair, Yokel's Hat, Knotted Shirt, Peasant Gloves, Peasant Pants, & Peasant Boots. If there is an option to dye the secondary color do this as well. Finally, I bought all of these items at the clothes store in the Bowerstone Market. The achievement will pop as soon as you exit back to the game, if you've hit everything.