向该对象的每一面添加不同的颜色

(function onLoad() {
  var canvasElement;
  var width, height;
  var scene, camera;
  var renderer;
  var controls;

  var pivot;
  var bagMesh;
  var planeMesh;
  
  const objLoader = new THREE.OBJLoader2();
  const fileLoader = new THREE.FileLoader();
 
  init();

  function init() {
    container = document.getElementById('container');
    initScene();
    addGridHelper();
    addCamera();
    addLighting();
    addRenderer();
    addOrbitControls();

    loadPlaneObj();
    
    // Logic
		var update = function() {};

		// Draw scene
		var render = function() {
			renderer.render(scene, camera);
		};

		// Run game logic (update, render, repeat)
		var gameLoop = function() {
			requestAnimationFrame(gameLoop);
			update();
			render();
		};
		gameLoop();
  }

  /**** Basic Scene Setup ****/
  function initScene() {
    scene = new THREE.Scene();
    scene.background = new THREE.Color(0xd3d3d3);
    var axis = new THREE.AxesHelper();
    scene.add(axis);
  }

  function addCamera() {
    camera = new THREE.PerspectiveCamera(70, window.innerWidth / window.innerHeight, 1, 1000);
    camera.position.set(3,3,3);
    scene.add(camera);
  }

  function addGridHelper() {
    var planeGeometry = new THREE.PlaneGeometry(2000, 2000);
    planeGeometry.rotateX(-Math.PI / 2);

    var planeMaterial = new THREE.ShadowMaterial({
      opacity: 0.2
    });
    var plane = new THREE.Mesh(planeGeometry, planeMaterial);
    plane.position.y = -200;
    plane.receiveShadow = true;
    scene.add(plane);

    var helper = new THREE.GridHelper(2000, 100);
    helper.material.opacity = 0.25;
    helper.material.transparent = true;
    scene.add(helper);

    var axis = new THREE.AxesHelper();
    scene.add(axis);
  }

  // *********** Lighting settings **********************
  function addLighting() {
    var light = new THREE.HemisphereLight(0xffffff, 0xffffff, 1);
    scene.add(light);
  }

  // ************** Material settings **************
  function setMaterial(materialName) {
    // get the object from the scene
    var bagMesh = scene.getObjectByName('bag');
    var material;

    if (!materialName) {
      materialName = materials.material;
    }

    if (bagMesh) {
      var colour = parseInt(materials.colour);
      switch (materialName) {
        case 'MeshBasicMaterial':
          material = new THREE.MeshBasicMaterial({
            color: colour
          });
          break;
        case 'MeshDepthMaterial':
          material = new THREE.MeshDepthMaterial();
          break;
        case 'MeshLambertMaterial':
          material = new THREE.MeshLambertMaterial({
            color: colour
          });
          break;
        case 'MeshNormalMaterial':
          material = new THREE.MeshNormalMaterial();
          break;
        case 'MeshPhongMaterial':
          material = new THREE.MeshPhongMaterial({
            color: colour
          });
          break;
        case 'MeshPhysicalMaterial':
          material = new THREE.MeshPhysicalMaterial({
            color: colour
          });
          break;
        case 'MeshStandardMaterial':
          material = new THREE.MeshStandardMaterial({
            color: colour
          });
          break;
        case 'MeshToonMaterial':
          material = new THREE.MeshToonMaterial({
            color: colour
          });
          break;
      }
      bagMesh.children.forEach(function(c) {
        c.material = material;
      });
    }
  }

  function setMaterialColour(colour) {
    materials.colour = colour;
    setMaterial(null);
  }
  // ************** End of materials ***************

  function addRenderer() {
    renderer = new THREE.WebGLRenderer({
      antialias: true
    });
    renderer.setPixelRatio(window.devicePixelRatio);
    renderer.setSize(window.innerWidth, window.innerHeight);
    renderer.shadowMap.enabled = true;
    container.appendChild(renderer.domElement);
  }

  function addOrbitControls() {
    var controls = new THREE.OrbitControls(camera, renderer.domElement);
  }

  function addPivot() {
    var cubeGeo = new THREE.BoxBufferGeometry(5, 5, 5);
    var cubeMat = new THREE.MeshBasicMaterial();
    pivot = new THREE.Mesh(cubeGeo, cubeMat);
    bagMesh.position.x -= 15;
    bagMesh.position.z -= 55;

    pivot.add(bagMesh);
    pivot.add(handle);
    scene.add(pivot);
  }
  
  function loadPlaneObj() {
		loadObj('Plane', 'https://rawgit.com/Katana24/threejs-experimentation/master/models/Plane.obj', 'https://rawgit.com/Katana24/threejs-experimentation/master/models/Plane.mtl', addPlaneToSceneSOAnswer);
	}
  
  function loadObj(objName, objUrl, mtlUrl, onLoadFunc) {
		var onLoadMtl = function(materials) {
	    objLoader.setModelName(objName);
	    objLoader.setMaterials(materials);

	    fileLoader.setPath('');
	    fileLoader.setResponseType('arraybuffer');
	    fileLoader.load(objUrl,
	      function(onLoadContent) {
	        var mesh = objLoader.parse(onLoadContent);
	        onLoadFunc(mesh);
	      },
	      function(inProgress) {},
	      function(error) {
	        throw new Error('Couldnt load the model: ', error);
	      });
	  };
	  objLoader.loadMtl(mtlUrl, objName+'.mtl', onLoadMtl);
	}
  
  function addPlaneToSceneSOAnswer(mesh) {
		var frontMaterial = new THREE.MeshBasicMaterial( { color : 0xff0000, side: THREE.FrontSide } );
		var backMaterial 	= new THREE.MeshBasicMaterial( { color : 0x00ff00, side: THREE.BackSide } );
		
		var geometry = new THREE.Geometry().fromBufferGeometry( mesh.children[0].geometry );
		var length = geometry.faces.length;
		geometry.faces.splice(14, 1);

		for (var i = 0; i < geometry.faces.length; i ++ ) {
			var face = geometry.faces[i];
			face.color.setHex(Math.random() * 0xffffff);
		}
		mesh = new THREE.Mesh( geometry, new THREE.MeshBasicMaterial({ vertexColors: THREE.FaceColors, side: THREE.DoubleSide }) );
		mesh.material.side = THREE.FrontSide; 
		
		var mesh2 = new THREE.Mesh( geometry, mesh.material.clone() ); 
		mesh2.material.side = THREE.BackSide; 
		// mesh2.material.vertexColors = THREE.NoColors; 
		mesh2.material.vertexColors = [new THREE.Color(0xff0000), new THREE.Color(0x00ff00), new THREE.Color(0x0000ff)];

		mesh.add( mesh2 );
		scene.add(mesh);
	}
})();

body {
  background: transparent;
  padding: 0;
  margin: 0;
  font-family: sans-serif;
}

#canvas {
  margin: 10px auto;
  width: 800px;
  height: 350px;
  margin-top: -44px;
}

<body>
  <div id="container"></div>
  <script src="https://threejs.org/build/three.js"></script>
  <script src="https://threejs.org/examples/js/libs/dat.gui.min.js"></script>
  <script src="https://threejs.org/examples/js/controls/OrbitControls.js"></script>
  <script src="https://threejs.org/examples/js/loaders/MTLLoader.js"></script>
  <script src="https://rawgit.com/mrdoob/three.js/dev/examples/js/loaders/LoaderSupport.js"></script>
  <script src="https://rawgit.com/mrdoob/three.js/dev/examples/js/loaders/OBJLoader2.js"></script>
</body>