Here is my xacro file:

<?xml version="1.0" ?>

<robot name="$(arg roboname)" xmlns:xacro="http://www.ros.org/wiki/xacro">

  <xacro:property name="PI" value="3.1415926835897931"/> 

<link name="base_link">
     <collision>
         <origin xyz="0 0 1" rpy="0 0 ${-PI/2}"/>
           <geometry>
            <box size="2.62 1.3 1.4478"/>
        </geometry>
      </collision>
    <gravity>0</gravity>
     <visual>
        <origin xyz="0 0 1" rpy="0 0 ${-PI/2}"/>
        <geometry>
            <box size="2.62 1.3 1.4478"/>
        </geometry>
         <material name="Yellow"/>
     </visual>

</link>

<joint name="inertial_joint" type="fixed">
      <parent link="base_link"/>
      <child link="main_mass"/>
      <origin xyz="0 0 0" rpy="0 0 0"/>
</joint>

<link name="main_mass" type="fixed">
    <!-- <parent link="inertial_joint"/> -->
     <gravity>0</gravity>
     <inertial>
        <origin xyz="0 0 1.0639" rpy="0 0 0"/>
        <mass value="1"/>
        <inertia
          ixx="1"  ixy="0.000000" ixz="0"
          iyy="0" iyz="0.000000"
          izz="0"/>
      </inertial>
</link>

</robot>

I want box to be of yellow color but I can't see in Yellow color in gazebo. How should I fix it?