For a dull metallic look like for cast iron, one can also mix steel or aluminum colors of regular hobby paint with a flat gray. No need to use Alclad. Also, sometimes I paint a part first with flay medium or dark gray, then drybrush aluminum or steel on it for a weathered steel patina.
For the shiny Alclad colors the Alclad must be applied in a very thin coat, and the Alclad itself is very thin, so mixing anything with those Aclads doesn't work very well.