I'd hate to argue with JP (again), especially on an electrical matter, (there's always a first time) but I'd be suspicious about the heat theory. The coloration on the set screw does not look like heat discoloration to me. And there's no melting on the plastic nib sticking through the bar below the set screw -- or anywhere else on the standoff. I could be wrong, but I seem to remember seeing a reddish factory coloration on some GE set screws. Almost a semi-transparent look, like someone wiped a magic marker over it.

As for the bar itself, even brand new, sometimes the plating on the bars is a bit multicolored. I think at most all you're seeing is a reaction between the plating on the bar and the environment.