What about this sample? It creates the material array and then adds it to the mesh. So, you could re-use it.
Relevant code:
// Create an array of materials to be used in a cube, one for each side
var cubeMaterialArray = [];
// order to add materials: x+,x-,y+,y-,z+,z-
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0xff3333 } ) );
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0xff8800 } ) );
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0xffff33 } ) );
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0x33ff33 } ) );
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0x3333ff } ) );
cubeMaterialArray.push( new THREE.MeshBasicMaterial( { color: 0x8833ff } ) );
var cubeMaterials = new THREE.MeshFaceMaterial( cubeMaterialArray );
// Cube parameters: width (x), height (y), depth (z),
// (optional) segments along x, segments along y, segments along z
var cubeGeometry = new THREE.CubeGeometry( 100, 100, 100, 1, 1, 1 );
// using THREE.MeshFaceMaterial() in the constructor below
// causes the mesh to use the materials stored in the geometry
cube = new THREE.Mesh( cubeGeometry, cubeMaterials );