Change the colors of a cube's faces

Here is how you set and change the colors of a cube's faces:

var geometry = new THREE.BoxGeometry( size, size, size );
for ( var i = 0; i < geometry.faces.length; i ++ ) {
    geometry.faces[ i ].color.setHex( Math.random() * 0xffffff );
}

var material = new THREE.MeshBasicMaterial( { color: 0xffffff, vertexColors: true } );

If geometry.faces[i].color is changed after the geometry has been rendered, you must set geometry.colorsNeedUpdate = true. ( This is not required for canvasRenderer. )

three.js r.84

Here is a fiddle for people who end up here and want to see this code working.

I made a box and tied 3 colors to the faces:

// colors
red = new THREE.Color(1, 0, 0);
green = new THREE.Color(0, 1, 0);
blue = new THREE.Color(0, 0, 1);
var colors = [red, green, blue];

for (var i = 0; i < 3; i++) {
    geometry.faces[4 * i].color = colors[i];
    geometry.faces[4 * i + 1].color = colors[i];
    geometry.faces[4 * i + 2].color = colors[i];
    geometry.faces[4 * i + 3].color = colors[i];
}

The face colors are changed in the animate loop.

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Also check a related question here with a great answer demonstrating the use of material indices and groups on THREE.BufferGeometry instances.