if you did need a dark metallic, you could overprint aka "spectratone" the Grey-SpectraShade (GCX-111) on Silver (GCS-620) and get a nice dark gray metallic... or variants of the same principle; print a translucent gray on a silver metallic vinyl.
another method is regular dark gray prints, then laminate with a specialty film that has a metallic flake in the laminate. there are a handful of special effects laminates out there.
i have made some very cool metallics with the 1st method on my Gerber Edge and translucent colors over printed on silver or gold.