Well, if shiny is easy to see, then why did the Army use tinned cases?
I don't think visibility of nickel vs. brass ever seriously entered into the equation. If Springfield or Frankford Arsenal had ever found any advantage to nickel plating, G.I.s would be loading shiny ammunition.
Another one of those myths, like hearing the ping of an empty clip being ejected from an M1 rifle. Good story, but not true.