I'll also add my vast experience *lol*
Here in Germany there isn't much choice when it comes to metallic colors for food.
1. Rainbow Dust pre-mixed metallics - they come in a vast range of colors, little bottles with 25ml, very expensive, to be applied with a brush. The results are so-so. They tend to get streaky (is there a word like that?), as they dry very quick and especially if you cover vast areas they dry faster than you can paint. I hardly ever use them because they are hard to control. Here is an example of what they look like (though the picture doesn't show the blotches, they were horrible in reality!), used on caps of mason jars and bottom of cake stands: Rainbow dust metallic paint
2. PME sprays - awesome results (sorry, I have no example...), but horrible price. I have never bought one myself because it is soooo expensive, only used it in classes I took. And it is hardly any use if you want to have small accents, e.g. gold beads, gold peaks for birds. It is meant for larger areas.
3. Rainbow dust powder metallics - you can either dust them on, which gives a nice and very even sheen (like this: Rainbow dust powder brushed on), but no real metallic effect, or mix them with vodka/clear extract and paint them on. If mixed to a paste consistency, they cover pretty good, but get blotchy, especially for larger areas (see here: Rainbow dust extended powders large area). If they are thinned down more, they are perfect for accents though, don't have streaks or blotches, and I use them quite frequently (e.g. here Rainbow dust extended powders thin consistency gold and here Rainbow dust extended powders thin consistency silver. But in thin consistency they are no good for covering large areas.
Generally, I find the results turn out a lot better if I use an icing in the same color I want to apply metallic colors later on, e.g. grey for silver, brown for gold. The coverage gets a lot better, forgotten spots don't show that much, and I need a lot less of the paint.