I just posted a thread yesterday asking about the differences of the dusts...found some info on the sugarcraft website that helped.
Pearl, Petal, Luster and Sparkle Dusts contain only ingredients that are non-toxic and are ideal for use on gumpaste or rolled fondant flowers, plaques, lettering, etc. They present a range or creative color possibilities and the opportunity to achieve color of unique liveliness and impact. These dusts are NOT a food additive and should not be considered as such. Intended for use in the dry form, any of the powders may also be mixed with oil-based flavorings or alcohol for painting and highlighting. For a more permanent application, liquefy with Tylose gum glue, or confectioners glaze. Each type of dust creates a different effect. Please note that colors indicated by * (in catalogs) contain Iron Blue or Chromium Oxide and MUST be labeled "for decorative use only".
Petal Dust is used to achieve deeper hues with a matte finish. When brushed on petals of white or pastel gumpaste flowers, a realistic effect is created. Pearl or luster dusts can be applied over or mixed with petal dust for a variety of effects. Mix with water, alcohol or oil-based flavorings for painting, highlighting or creating deeper colors. Contains no cornstarch. Colors will intensify when mixed with liquids. Certified non-toxic. Water soluble.
Luster Dust gives subtle color with a high sheen metallic-like finish. They may be used alone or mixed with similar
colored matte dusts to create a lustrous sheen without lightening the color. Brushed on white, they exhibit color and metallic-like sheen. Contains two or more of the following - Titanium Dioxide, Iron Oxide, Carmine and Mica. Not
Sparkle dust is similar to Luster Dust in effect and use but is a larger grain size. Gives gumpaste flowers a wed to dewy look, with just a touch of color. Contains two or more of the following - Titanium Dioxide, Iron Oxide, Carmine and
Mica. Not water soluble.
Pearl dust produces a shimmery luster. Use super pearl on flowers made from colored gum paste for a glistening look. When dusted on white petals, the other pearls in the range exhibit a subtle translucent or iridescent reflective colored sheen while displaying no apparent or strong color. They may be combined or applied over Petal dust to create custom colors or effects. Contains Titanium Dioxide and Mica. Not water soluble.