Yharnam Vestments is a dye for Kensei Armor. You earn it by completing the Blood-Stained Shrine, one of the Wind Shrines that reward special armor dyes. Like every dye it only changes colours.
Find the Blood-Stained Shrine and clear it. The dye is part of the reward, together with a piece of vanity gear from the same shrine.
Once unlocked it shows up as a colour option for Kensei Armor, so you need that outfit to use it.
Adventurer's Fortune
Adventurer's Fortune is a dye for Traveler's Attire. It costs 10 flowers at the Izuhara merchants. Like every dye it only changes colours.
Amber Assassin
Amber Assassin is a dye for Ronin Attire. It costs 10 flowers at the Toyotama merchants. Like every dye it only changes colours.
Ancestral Force
Ancestral Force is a dye for Sakai Clan Armor. It costs 10 flowers at the Toyotama merchants. Like every dye it only changes colours.
Archery Master's Attire
Archery Master's Attire is a dye for Tadayori's Armor. It is a New Game+ dye sold by Baku the Voiceless for 15 Ghost flowers. Like every dye it only changes colours.