Yellow Chrysanthemum is a dye for the Sakai Horse Armor. Tsubaki gives it to you once you have collected 12 Sakai Banners on Iki Island. Like every dye it only changes colours.
Pick up Sakai Banners as you explore Iki. After 12 banners, visit Tsubaki to claim this dye. A new horse dye unlocks every 6 banners, up to 30.
Once unlocked it shows up as a colour option for the Sakai Horse Armor, so you need that outfit to use it.
A Scrawled Message
A Scrawled Message is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a camp south of Lake Nagata on Iki Island, and it counts toward the Avid Reader trophy.
Attached to an Old Crate
Attached to an Old Crate is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a shed northwest of Lone Spirit Fall on Iki Island, and it counts toward the Avid Reader trophy.
Image: Sucker Punch Productions / PlayStationBamboo Strikes
Bamboo Strikes are button-timing challenges where Jin slices a row of bamboo poles. There are 20 in all, 16 on Tsushima and 4 on Iki, and completing them raises your maximum Resolve.
Beware the Tengu-Masked Archer
Beware the Tengu-Masked Archer is a written record from the Records of Iki set, left by locals during the Mongol invasion. You find it at a house in Yahata Forest on Iki Island, and it counts toward the Avid Reader trophy.