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

{
  "asset_name": "diamond",
  "description": {
    "color": "#6EECD2",
    "translate": "trim_material.minecraft.diamond"
  },
  "ingredient": "minecraft:diamond",
  "override_armor_assets": {
    "minecraft:diamond": "diamond_darker"
  }
}