Metal leaf pens (or "leafing pens") come in a fairly limited number of colors, but they can be very useful for adding a gilded touch to polymer clay projects.
Leafing pens are best used to highlight specific elements of a design or to jazz up a piece with a few carefully placed embellishments. You can also use them to gild carved lines. For total coverage of a piece, you may find that metal leaf, liquid leaf, or metallic paints are more economical.
You may use metal leaf pens on raw or cured clay, but curing the metal leaf ink may cause it to change colors. If the surface will get a great deal of wear, it's a good idea to seal it. Some sealants may smear the ink, though, so let the piece dry very thoroughly (for days, even) before sealing it, and use a light touch with the brush.