How to add a color/material to a STL mesh in an URDF file?

edit

You need to define the color separately such as in the file materials.xacro:

  <material name="Orange">
    <color rgba="${255/255} ${108/255} ${10/255} 1.0"/>
  </material>

Import materials.xacro in your file:

<xacro:include filename="$(find your_robot_description)/urdf/materials.xacro" />

Then you can use the color specifying a material tag for each link, such as:

<link name="$link_name">
  ...
  <visual>
     ...
    <material name="Orange"/>
  </visual>
  ...
</link>

For further information visit: http://gazebosim.org/tutorials/?tut=r...

Previous answers are actually correct, the trick here is not to provide a name for the "visual". e.g.:

Do not:

<visual name="foo"> <!-- gazebo will ignore your color request -->
...
</visual>

Do instead:

<visual>
...
</visual>

Apart from that, you need to provide appropriate gazebo tags, I will leave a full working example below:

<?xml version="1.0"?>
<robot name="my_robot_name">

  <!-- this color take effect only on rviz -->
  <material name="WHITE_MTL">
    <color rgba="0.0 1.0 0.0 1.0"/> <!-- rviz green color -->
  </material>

  <link name="my_link_name">
    <inertial>
      <origin xyz="0 0 1" rpy="0 0 0"/>
      <mass value="0.5"/>
      <inertia ixx="0.002" ixy="0" ixz="0" iyy="0.002" iyz="0" izz="0.002"/>
    </inertial>
    <visual> <!-- do not give a name! otherwise gazebo wont pick the color -->
      <origin xyz="0 0.0 0" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://my_robot_description/meshes/stl/my_file.stl" scale="1.0 1.0 1.0"/>
      </geometry>
      <material name="WHITE_MTL"/> <!-- rviz will use this color -->
    </visual>
    <collision name="my_collision1">
      <origin xyz="0 1.0 0" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.5" length="0.2"/>
      </geometry>
    </collision>
    <collision name="my_collision2">
      <origin xyz="0 -1.0 0" rpy="0 0 0"/>
      <geometry>
        <cylinder radius="0.1" length="0.3"/>
      </geometry>
    </collision>
  </link>

  <gazebo reference="my_link_name"> <!-- reference to a existing "link" -->
    <!-- NOTE: use Gazebo/DepthMap to take color from mesh (.dae only) -->
    <material>Gazebo/Orange</material>
    <mu1>0.2</mu1>
    <mu2>0.2</mu2>
    <selfCollide>True</selfCollide>
  </gazebo>

  <!-- fix arm base, so that it does not fall -->
  <link name="world"/>
  <joint name="world_to_robot" type="fixed">
    <parent link="world"/>
    <child link="my_link_name"/>
    <origin xyz="0 0 0.5" rpy="0 0 0"/>
  </joint>

</robot>