<gazebo reference=""> not loaded from urdf.xacro Gazebo6

edit

Hello Everyone,

Using Gazebo6.6, Ros Indigo.

I am using a .xacro to generate my model spawn it via the spawn model plugin.

Lately i was trying to change the colors of my vehicle in the simulation. But for some reason none of my tries resulted in a change of color in my simulation. I definied a reference for each link but still no results.

Here is how i load the gazebo tag:

<gazebo reference="wheel_f_l">
     <material>
          <script>Gazebo/Black</script>
        </material>
    </gazebo>

Even with:

<gazebo reference="wheel_f_l">
     <material>Gazebo/Black</material>
    </gazebo>

it doesn't work. Though setting the colors for visualization via rgba in Rviz works. Did anybody experience sth. similar. It should work, because i saw many examples on github etc. Did i miss something? Help would be very appriciated.

EDIT: some of my launch file

<launch>

<...>

  <!-- start gazebo server-->
  <node name="gazebo" pkg="gazebo_ros" type="gzserver" respawn="false" output="screen" args="$(arg command_arg1) $(arg world_name) --verbose" />

      <!-- load vehicle description to the parameter server -->
  <param name="robot_description" command="$(find xacro)/xacro.py '$(arg robot_model)' tf_prefix:='$(arg tf_prefix)' ns:='$(arg robot_ns)'" />

  <group ns="$(arg robot_ns)">

    <!-- load vehicle description to the parameter server -->
    <param name="robot_description" command="$(find xacro)/xacro.py '$(arg robot_model)' tf_prefix:='$(arg tf_prefix)' ns:='$(arg robot_ns)'" />

    <!-- spawn vehicle model in gazebo -->
    <node name="spawn_robot_model" pkg="gazebo_ros" type="spawn_model"
      args="-param robot_description
      -urdf
      -x $(arg pos_x)
      -y $(arg pos_y)
      -z $(arg pos_z)
      -Y $(arg yaw)
      -model $(arg robot_name)"
      respawn="false" output="screen"/>


</launch>