Different material on back and frontside of extruded shape

I'm trying to apply different material on front and back sides of extruded shape, but cannot figure out where to put side: THREE.FrontSide and side: THREE.BackSide. Where they should be putted?

My relevant code part is:

var materialFront = new THREE.MeshPhongMaterial({ ambient: 0xffffff, map: frontTexture });

var materialSide = new THREE.MeshPhongMaterial({color: 0xE68A00, ambient: 0xffffff});

var extrusionSettings = {
          amount: 10,
          bevelEnabled: false,
          bevelThickness: 0.2,
          bevelSize: 0.2,
          bevelSegments: 8,
          material: 0,
          extrudeMaterial: 1
};

var geometry = new THREE.ExtrudeGeometry(shapes, extrusionSettings);

var materials = [materialFront, materialSide];

var material = new THREE.MeshFaceMaterial(materials);

mesh = new THREE.Mesh(geometry, material);

UPDATE: According to WestLangley's comment I succeeded in adding the different texture to backfaces:

// ...
var materials = [materialFront, materialSide, materialBack];
// ...
for ( var face in mesh.geometry.faces ) {
if (mesh.geometry.faces[ face ].normal.z == 1) mesh.geometry.faces[ face ].materialIndex = 2;
}