You can store the clicked model in a global variable outside the onClick function, to change it back if you have not clicked the model.
var object // Global variable
onClick() {
....
if (intersects.length > 0) {
object = intersects[0].object;
object.material.color.set( Math.random() * 0xffffff );
}
else { // stored object -> change back to #FFFFFF
object.material.color.set( 0xffffff );
}
}