minecraft-assets/data/minecraft/recipes/gray_dye.json

{
  "type": "minecraft:crafting_shapeless",
  "ingredients": [
    {
      "item": "minecraft:black_dye"
    },
    {
      "item": "minecraft:white_dye"
    }
  ],
  "result": {
    "count": 2,
    "item": "minecraft:gray_dye"
  }
}