If rubber stamping polymer clay I use Paulette Hybrid stamp Ink by Stuart Superior Corporation. The ink stays very well on polymer clay, baked polymer clay, and keeps it's good colors. I like to buy the inks and sets of four colors that are collected by the company and assigned to a specific artist.
I have most of the four color "Artist" sets. They are small and can be left open when inking as they do not dry out quickly.
For example the Escher palette has these colors; New Canvas, Burnt Number, Charcoal, Noir. There website does not sell the inks but they do have a list of dealers who sell the ink.My favorite color of these four is New Canvas. It is a pale ivory color and looks good on dark clay.
None of these colors are metallic but they are excellent when I'd like to add detail to a project.