You can also airbrush pearl lusters on cakes. I just saw one this weekend that had lavender roses and an ivory luster sprayed on them. it was stunning. These are available at your local bakery supply store and are in a variety of colors.
Christine, I hope this is not too late to help you. I did a wedding cake last weekend in which I used both luster dust and pearl dust on buttercream frosting. I don't know what colors you are working with, but I was trying to achieve the look of shimmering gold on the cake. I did a lot of experimenting, and here's what I found: in my opinion the luster dust looked better on plain white or ivory-colored frosting. I used "old gold" luster dust on an off-white frosting. I waited for the buttercream to crust some, then using a tiny dry brush I very very carefully brushed the luster dust on my decorations. It accentuated the leaves and flowers and shell decorations.
Pearl dust is very striking. It gives a very subtle glow to the cake. I used "gold pearl" pearl dust. It looks pretty also if you brush the pearl dust on top of the luster dust. I think I liked the pearl dust better than luster dust. You can mix the pearl dust and luster dust together before applying it to the cake. That softens the color of the luster dust.
I hope you have time to experiment before you have to do the final cake. Be careful if you are applying the luster dust to colored frosting. I made that mistake. I had colored the decorating frosting what I thought was a pale, pale gold, and it looked pretty until I brushed some "old gold" luster dust on it, and then all of a sudden it seemed to resemble deviled eggs. So if you can, experiment on a test cake first. As I already mentioned, I thought it looked best on non-colored frosting.
If you have the time, I'd like to hear what you did, and how you liked the results. This was my first time using this stuff. Hope this has helped you.
Babzee Thank you very much for the information. I'm very new to decorating and I was a little confused about the different kinds of dusts. I think most of what I have is Wiltons pearl dust. It looks to me as though all of those are edible. I'm making the cake next week and she wants something simple but feminine so I was thinking small beads of buttercream using different colors of dust in pastels on a smooth white cake. I tried to get her to go with fondant but like most people I know her first response was Yuk. I love working with Fondant but I can't say I blame her, there is nothing like a good buttercream. Once again I appreciate the information, I got it in plenty of time and if I get gutsy enough I will post a picture after it is complete.