First, some paints and clear finishes and also some of the propellants in sprays will interact with polymer clay if applied directly to them, and begin dissolving the clay over time.
Real "chrome" is usually a color that requires using a paint that will be one of the bad ones though, but you can always coat the hardened clay with a water-based paint or clear finish THEN use one of the "bad" types on top of that (since they won't then be in direct contact with the clay).
Many kinds of "silver" paint and other silver colorants can be used safely on polymer clay however.
For example, "imitation leaf" (silver in this case, or actually zinc) or "real-metal" powder would give quite chrome-y effects (high shine) but those aren't really "paints."
If you used imitation leaf, you'd generally apply it to the uncured clay since it will stick wonderfully well (all over if that's what you want), then bake. If you use "silver" leaf, you won't need to use a clear finish to prevent oxidation as with "gold," etc, but you could if you wanted to.
If you used metal powder (use a mask for this kind of metal powder), you'd generally brush it onto raw polymer clay before baking, but you could actually mix it into a clear finish of some kind and use it as a paint (after baking). Again, some of those real-metal powders would need sealing from oxidation, though most metallic *mica* powders (which aren't as "hard" a shine, and not bad for lungs too) wouldn't.
There are also metallic (acrylic) "inks" you might use, as well as metallic pens (Krylon, etc...though some aren't safe directly on polymer clay), and metallic waxes (usually not really high shine), etc.
Check out those materials and techniques from these pages at my polymer clay "encyclopedia" site to use on your skull casts (or on any polymer clay):
http://www.glassattic.com/polymer/Faux--many.htm < Metals
http://www.glassattic.com/polymer/leaf.htm
http://www.glassattic.com/polymer/powders_metallicwaxes.htm
http://www.glassattic.com/polymer/paints.htm
http://www.glassattic.com/polymer/letters_inks.htm
...and some of my previous answers may have summaries about those things:
https://answersrip.com/question/index?qid=20120515015152AAi1WJj
(It would help if you could say just why your results didn't look the way you wanted too.)
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