minecraft-assets/assets/minecraft/items/wither_skeleton_spawn_egg.json

{
  "model": {
    "type": "minecraft:model",
    "model": "minecraft:item/template_spawn_egg",
    "tints": [
      {
        "type": "minecraft:constant",
        "value": -15461356
      },
      {
        "type": "minecraft:constant",
        "value": -12104371
      }
    ]
  }
}