There's a separate post explaining how to get a smooth effect with lustre/metallic dusts here.

Are dust colours edible?

Dust food colours are edible. Some cake decorating shops sell non-toxic, non-edible dust colours too. Those are usually very vibrant colours or metallic colours. That doesn’t mean that all vibrant and metalic dusts are inedible. Only some of them. So be cautious when choosing the products you buy.

Non-toxic, non-edible dust colours can be used for parts that you know won’t be eaten, like for example fake dummy tiers.