I've been asked to make a cake using metallic purple as one of the colours. I assume that they're happy for that to be a small part of the colour scheme. Any recommendations for how to achieve that? Is there a luster dust that might be helpful? Would mixing silver dust with purple achieve that kind of look?

Any suggestions would be appreciated!

I don't know what stores you have avail or time to order online, but there is a purple pearl dust you can buy at Michaels (or probably order from either hobby lobby or wilton online).

Just add a little vodka to it and you can paint or airbrush it. I'd recommend tinting your fondant purple so that it won't take too much liquid to achieve the color - just enough to get the sheen etc.

Anyway you may want to look for it. It's pretty.