Changing color of Model in Gazebo via ModelPlugin

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I'm currently working on an urban city world for Gazebo Simulator on ROS Indigo and have a problem.

I want to create a Model Plugin, that controls Traffic lights. So I created a model where every Lightbulb is a Link.

TrafficLight::trafficlight_light_RED_link
TrafficLight::trafficlight_light_YELLOW_link
TrafficLight::trafficlight_light_GREEN_link

Then I created a ModelPlugin that should Manage the Colour of the lights.

#include <boost/bind.hpp>
#include <gazebo/gazebo.hh>
#include <gazebo/physics/physics.hh>
#include <gazebo/common/common.hh>
#include "ros/ros.h"
#include "std_msgs/String.h"
#include <stdio.h>


namespace gazebo
{
  class ModelPush : public ModelPlugin
  {
    public: void Load(physics::ModelPtr _parent, sdf::ElementPtr /*_sdf*/)
    {

        std::cout << "******* Loading Trafficlight Plugin **********" << std::endl;

      // Store the pointer to the model
      this->model = _parent;

      // Listen to the update event. This event is broadcast every
      // simulation iteration.
      this->updateConnection = event::Events::ConnectWorldUpdateBegin(
          boost::bind(&ModelPush::OnUpdate, this, _1));

     this->link_vector = this->model->GetLinks();

    for (int i=0;i<this->link_vector.size();i++){

        std::cout << this->link_vector.at(i)->GetScopedName() << std::endl;     
    }

   }

    // Called by the world update start event
    public: void OnUpdate(const common::UpdateInfo & /*_info*/)
    {
      // Apply a small linear velocity to the model.

    }

    // Pointer to the model
    private: physics::ModelPtr model;

    // vector for Model Links
    private: physics::Link_V link_vector;


    // Pointer to the update event connection
    private: event::ConnectionPtr updateConnection;
  };

  // Register this plugin with the simulator
  GZ_REGISTER_MODEL_PLUGIN(ModelPush)
}

I found a solution to change the colour of an object here

He used a VisualPlugin and therefore got a

gazebo::rendering::VisualPtr

and so he is able to change Ambient and Diffuse Maps of the Visual Object

But I only have a Pointer to a Model Object. Is there a way to change the colour of an Object via ModelPlugin ? Already searched gazebo-API but couldn't find a solution.

Thanks for your Help