When you are dragging a ravager spawn egg, it's color will become the mask spawn egg. Apparently the ravager.entity.json file uses "base_color" and "overlay_color" instead of "texture" and "texture_index". Here's an comparison between a ravager's spawn egg and a vindicator's spawn egg. This is seen in the ravager.entity.json file.

ravager.entity.json

  "geometry": {
        "default": "geometry.ravager"
      },
      "render_controllers": [
        "controller.render.ravager"
      ],
      "spawn_egg": {
        "base_color": "#757470",
        "overlay_color": "#5b5049"

vindicator.entity.json

  "geometry": { "default": "geometry.vindicator.v1.8" },
      "spawn_egg": {
        "texture": "spawn_egg",
        "texture_index": 39