我怎样才能对形状的正面和背面进行不同的着色？

Question

如何在形状的正面和背面涂抹不同的颜色？我的尝试：

var shape = new THREE.Shape( pts );
var geometry = new THREE.ExtrudeGeometry( shape, extrudeSettings );
var material2 = new THREE.MeshLambertMaterial( { color: 0x66CC00, wireframe: false } );
mesh = new THREE.Mesh( geometry, material2 );
scene.add( mesh );