How to paint colors to the assets cube through .urdf

1. Find your assets/cube.urdf

Something looks like this

<?xml version="1.0"?>
<robot name="object">
  <link name="object">
    <visual>
      <origin xyz="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.05"/>
      </geometry>
    </visual>

    <collision>
      <origin xyz="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.05"/>
      </geometry>
    </collision>

    <inertial>
      <mass value="0.05" />
      <inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/>
    </inertial>
  </link>
</robot>

2. Then you can paint colors onto the surface of your asset by change the material property in .urdf file

<?xml version="1.0"?>
<robot name="object">
  <link name="object">
    <!-- Front Face -->
    <visual>
      <origin xyz="0 0.025 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.001 0.05"/>
      </geometry>
      <material name="FrontMaterial">
        <color rgba="1 0 0 1"/> <!-- Red -->
      </material>
    </visual>

    <!-- Back Face -->
    <visual>
      <origin xyz="0 -0.025 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.001 0.05"/>
      </geometry>
      <material name="BackMaterial">
        <color rgba="0 1 0 1"/> <!-- Green -->
      </material>
    </visual>

    <!-- Left Face -->
    <visual>
      <origin xyz="-0.025 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.001 0.05 0.05"/>
      </geometry>
      <material name="LeftMaterial">
        <color rgba="0 0 1 1"/> <!-- Blue -->
      </material>
    </visual>

    <!-- Right Face -->
    <visual>
      <origin xyz="0.025 0 0" rpy="0 0 0"/>
      <geometry>
        <box size="0.001 0.05 0.05"/>
      </geometry>
      <material name="RightMaterial">
        <color rgba="1 1 0 1"/> <!-- Yellow -->
      </material>
    </visual>

    <!-- Top Face -->
    <visual>
      <origin xyz="0 0 0.025" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.001"/>
      </geometry>
      <material name="TopMaterial">
        <color rgba="1 0 1 1"/> <!-- Magenta -->
      </material>
    </visual>

    <!-- Bottom Face -->
    <visual>
      <origin xyz="0 0 -0.025" rpy="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.001"/>
      </geometry>
      <material name="BottomMaterial">
        <color rgba="0 1 1 1"/> <!-- Cyan -->
      </material>
    </visual>

    <!-- Collision (Single Element) -->
    <collision>
      <origin xyz="0 0 0"/>
      <geometry>
        <box size="0.05 0.05 0.05"/>
      </geometry>
    </collision>

    <!-- Inertial Properties -->
    <inertial>
      <mass value="0.05" />
      <inertia ixx="0.0001" ixy="0.0" ixz="0.0" iyy="0.0001" iyz="0.0" izz="0.0001"/>
    </inertial>
  </link>
</robot>