Rosparam fails silenntly with a wrong param?

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rosparam is not failing silently, it's doing exactly what you're telling it to. As ΦXocę 웃 Пepeúpa ツ said, you're actually setting the parameter to that value. According to the rosparam wiki, the syntax for setting a parameter via the command line is:

rosparam set parameter_name value

where, in your case, parameter_name is /background_r and value is "foo". If you were to call

rosparam get /background_r

after running the command that "silently fails" you would see that it actually has the value of "foo". The parameter server just stores a bunch of dictionary values that you can set, use, and even delete.