a photo (nicked from Wikipedia) of an actual bright blue gas: trifluoronitrosomethane, CF3C-N=O.
a bright blue gas! iodine vapor is purple, but there's something special about blue. obtained by pyrolysis of trifluoroacetyl nitrite, obtained by reaction of nitrosyl chloride with silver trifluoroacetate. it even has a use: reaction with tetrafluoroethylene or other perfluorinated olefins yields "nitroso rubbers" of unusual properties.
~Alyx