minecraft-assets/data/minecraft/trim_material/iron.json

{
  "asset_name": "iron",
  "description": {
    "color": "#ECECEC",
    "translate": "trim_material.minecraft.iron"
  },
  "ingredient": "minecraft:iron_ingot",
  "override_armor_assets": {
    "minecraft:iron": "iron_darker"
  }
}