diff drive example missing colors (backport #310)

[mergify]

when running ros2 launch gz_ros2_control_demos diff_drive_example.launch.py the following errors are seen:

[robot_state_publisher-2] Warning: link 'chassis' material 'orange' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'chassis' material 'orange' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'left_wheel' material 'black' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'left_wheel' material 'black' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'right_wheel' material 'black' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'right_wheel' material 'black' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'caster' material 'white' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp
[robot_state_publisher-2] Warning: link 'caster' material 'white' undefined.
[robot_state_publisher-2]          at line 85 in ./urdf_parser/src/model.cpp

and

[ruby $(which gz) sim-1] Warning: link 'chassis' material 'orange' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'chassis' material 'orange' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'left_wheel' material 'black' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'left_wheel' material 'black' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'right_wheel' material 'black' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'right_wheel' material 'black' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'caster' material 'white' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp
[ruby $(which gz) sim-1] Warning: link 'caster' material 'white' undefined.
[ruby $(which gz) sim-1]          at line 85 in ./urdf_parser/src/model.cpp

without the changes:

with the changes:

---

This is an automatic backport of pull request #310 done by [Mergify](https://mergify.com).