On this page of our trophy guide to Ghost of Tsushima you can find a detailed description of Monochrome Masters trophy. This trophy can be earned for purchasing any item from a white dye merchant and from a black dye merchant. The following description reveals where both unique traders can be found.
White Dye Merchant
White Dye Merchant can be found in the first region of the game world, Izuhara. Explore the area of Wanderer's Pass. The trader will stand at the base of the watchtower.
The trader only sells cosmetic items that can be bought for flowers. The latter can be found in the game world. To get this trophy simply buy any white dye for your armor.
Black Dye Merchant
Black Dye Merchant can be found in the second region of the game world, Toyotama. Explore the area around Kawamata Village. The trader resides inside a small cave.
As in the case of the White Dye Merchant you need to have some flowers if you want ot trade. If you have purchased a white dye beforehand, then you will automatically unlock the trophy.