thanks very much for all the comments everyone, I am very happy that the article has been interesting or useful to some of you!
As for gold metallics: well, the principle is exactly the same, I should have stressed that point more in this article. You can paint golds in exactly the same way, just by substituting gold metallic colours rather than the silvers I have used here. And it often helps to mix a bright silver colour with the gold metals for the final highlights, because the silver metallic paints tend to be brighter than the golds.