Petal Dusts are edible petal dusts are used to give a wonderful matte color to white or colored gumpaste, fondant, and even royal icing. The dusts can be applied dry with an artist's brush or you can use clear spirits (such as vodka) or lemon extract to dilute a small amount of dust for painting on gumpaste or fondant. If applied dry you should gently steam the gumpaste/fondant for approximately 5 seconds to set the color (so it doesn't come off on your hands or the cake surface). Be very careful not to over steam and don't touch the freshly steamed surface or you will mark it with your fingers.
Luster Dust is a shiny, metallic looking dust that can be brushed on dry to add some color and sparkle. To get a deep, shiny pearl or gold you mix them with clear spirits (such as vodka) and paint them onto the dried gumpaste. They can also be thinned with clear spirits and used in an airbrush to spray the color onto gumpaste. Clean your airbrush thoroughly after spraying so the Luster Dust doesn't clog the airbrush parts. Many cake artists will first use a petal dust and then overlay a luster dust on top.
Highlighter/Shimmer Dusts have sparkle and can be brushed on dry over white or even colored dried gumpaste. It will add sparkle without changing the color too much. You can also add clear spirits and paint the shimmer on the gumpaste, but it won't have the intensity of Luster Dust.
Pearl dust comes in different colors (blue pearl, green pearl, etc), and adds an iridescent glimmer rather than a deep color. The super pearl gives a definite shine over the regular pearl dust, especially if you paint it on with lemon extract or clear spirits. The platinum gives a duller silvery shine.
Non-toxic gold, silver, and Copper dusts look best when painted on with lemon extract or clear spirits (vodka for example). The painted gumpaste takes on the sheen and glow of metal.
Non-toxic chalks give a matte color to white or colored gumpaste/fondant. In the long run, Scott's non-toxic chalks last a really really long time--in my opinion, more bang for your buck--over the small vials of petal dusts. I crush the non-toxic chalks to the consistency of the petal dusts, mix up my own color combos, and then use them.
I have tried different brands of dusts from different suppliers and have used the non-toxic chalks sold by Scott Clark Woolley. The quality of the dusts is about the same. Whether you "paint" with lemon extract or vodka is a matter of preference. I have used both with great results.