There are various
choices for ways to get a metallic effect on polymer clay, and the clay underneath can be any color including but not limited to black.
It's just that a dark color or toned-down color will look more like old dirt and grime that's collected in the crevices of an old item, but any color that's toned down or darker than the "original" color of the item would have been when it was new, would work.**
That book was written by a person (in France) who's not primarily a polymer clayer though, so some of the info isn't what would be suggested by a clayer. It's more from the point of view of someone who works in theater, etc, who learned to use polymer clay for parts of some of the things she wanted to replicate and then uses other materials she's familiar with like paints, and who also uses only purchased stones, etc (many of which could also be quite convincingly made from polymer clay too--we call those "fauxs").
I haven't read the book but one review about it on my site indicates that she's recommending "model paint" (like Testors/etc) which would be a no-no on polymer clay unless the clay had first been sealed (paints and finishes that are petroleum-solvent based, and must be cleaned up with paint thinner/etc, can eventually begin dissolving polymer clay over time making it sticky or worse).
Model paint isn't the same material as artists' "oil paint" though which is not
petroleum-solvent based--it's linseed oil based instead, can take a long time to dry and isn't usually opaque but could be used for some effects (rebaking oil paint for a short time can help speed up its hardening).
Instead, when a metallic (or any other color) effect is desired to "highlight" (a technical term in polymer clay) just the top surfaces of a dimensional polymer clay item, a metallic acrylic
paint/ink could be applied, but much more often metallic powders
are the materials used (either mica-based powders like Pearl Ex, etc, or real-metal powders like the Mona Lisa ones), and sometimes metallic waxes (Gilder's Paste, Rub 'N Buff). Even real-metal leaf or metallic foils can be used for some things.
For loads more info on creating "aged" looks with polymer clay, "highlighting" and its opposite "antiquing," using paints/powders/waxes/etc, and also making faux gems and doing fancy "onlay" effects, check out the following pages (the categories indicated in particular) at my site for examples, lessons, techniques, etc.
Since this would be a bunch of links, just go to the Table of Contents
page at the site then click on each of the names of pages listed below from inside the alphabetical navigation bar on the left there:http://glassattic.com/polymer/contents.htm...Fauxs-Many Types > Metals and > Ancient & Aged Looks ...also various gems if you want
...Powders, Waxes--Metallic > Techniques for All Powders,
as well as Mica Powders, Real Metal Powders, Metallic Waxes
...Paints > Acrylic >> Pearlescent, Metallic
...Molds > Antiquing, Highlighting, Staining
...Onlay > Dimensional Onlay >> Ropey, Textured, Shaped things...Bezels...Bas Relief
You might also want to check out the Jewelry page > Miscellaneous >> Renaissance, etc., Jewelry, Crowns
**Or instead, the dimensional clay item could also be antiqued in its crevices as well as highlighted on its top surfaces, which would create a slightly different or completely different color in the crevices rather than just letting the original clay color show through unchanged.