If I may add some information, the rule "no metal on metal" is also true for colors. Apparently the rule was meant to increase contrast between the parts of the banner, so that it would be easily recognised from afar. Metals would have more "shiny" fabric, compared to plain colors. (white and yellow being fairer than the other colors, I guess? I don't think cloth-of-gold would be used for banners, though)
So I guess even when designing fantasy heraldry, one should still keep in mind that goal.
Nice cheat sheet!
Hi Pryme8, just renamed your title to make it a little clearer as to what this thread is about.