I want to do picking via IdMapping in Three.js

Because of performance issues I only have one huge geometry, computed like this:

for (var i = 0; i < numberOfVertices; i += 9) {

  p1  = new THREE.Vector3(graphData.triangles.vertices[i+0], graphData.triangles.vertices[i+1], graphData.triangles.vertices[i+2]);

  p2  = new THREE.Vector3(graphData.triangles.vertices[i+3], graphData.triangles.vertices[i+4], graphData.triangles.vertices[i+5]);

  p3  = new THREE.Vector3(graphData.triangles.vertices[i+6], graphData.triangles.vertices[i+7], graphData.triangles.vertices[i+8]);

  geometry.vertices.push(new THREE.Vertex( p1.clone() ));

  geometry.vertices.push(new THREE.Vertex( p2.clone() ));

  geometry.vertices.push(new THREE.Vertex( p3.clone() ));

  geometry.faces.push( new THREE.Face3( i/3, i/3+1, i/3+2 ) );



  // i want to do something like this:

  geometry.colors.push(new THREE.Color(0xFF0000));

  geometry.colors.push(new THREE.Color(0xFF0000));

  geometry.colors.push(new THREE.Color(0xFF0000)); 

}



geometry.computeFaceNormals();

var material = new THREE.MeshBasicMaterial({});



var triangles = new THREE.Mesh( geometry, material );

scene.add(triangles);

How can I assign different colors to each vertex in my geometry?