We can see in the simulated robot that each link has different colors and textures. The following tags inside the xacro file provide textures and colors to robot links:
<gazebo reference="bottom_link"> <material>Gazebo/White</material> </gazebo> <gazebo reference="base_link"> <material>Gazebo/White</material> </gazebo> <gazebo reference="shoulder_pan_link"> <material>Gazebo/Red</material> </gazebo>