minecraft-assets/assets/minecraft/models/item/leather_boots_copper_trim.json

{
  "parent": "minecraft:item/generated",
  "textures": {
    "layer0": "minecraft:item/leather_boots",
    "layer1": "minecraft:item/leather_boots_overlay",
    "layer2": "minecraft:trims/items/boots_trim_copper"
  }
}