Midori Spring is a dye for the Sakai Horse Armor. Tsubaki gives it to you once you have collected 6 Sakai Banners on Iki Island. Like every dye it only changes colours.
Pick up Sakai Banners as you explore Iki. After 6 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.