The two merchants are in Inzuaha and Toyotama.
White Dye Merchant
In the mountains of Wanderer's Pass. West of the Wanderer's Pass mark on the games map and directly south of Kashine Bamboo Strike.
Black Dye Merchant
In a cave slightly North-west of Yarikawa Stronghold and almost on top of the E of the words Kawamata Village
Buy one item from each merchant fof the trophy.