<launch>
<rosparam file="$(find turtlesim_launch)/launch.yaml" command="load" />
<node name="turtlesim_node" pkg="turtlesim" type="turtlesim_node" respawn="false" output="screen" />
<rosparam file="$(find turtlesim_launch)/launch.yaml" command="load" />
</launch>

and my launch.yaml is

background_r: 0
background_g: 0
background_b: 0

I also use rosservice call /clear but it does not work