Can luste dust be used to colour royal icing
If you're thinking of Lustre/Luster dust. It's mainly used as a topping of some sorts, kinda like a spray of powder on top of royal icing, I believe you can use it to color royal icing. I've never done it personally, but I've watched certain videos wherein lustre dust is, indeed used to color royal icing.
Absolutely. Just make sure the royal icing is dry/hard. The dust can be brushed on dry or mixed with grain alcohol or lemon oil to make a paint.