Can anyone tell me if it's possible to mix metallic luster dust into the RI when doing scroll work or does it need to be painted on afterwards?

it works better painting it on afterwards.
xx

If you mix the 2 it will not come out shiny.I have seen someone mix luster with piping gel to do scroll work and it came out beautifully.But yes I think painting it afterwards might be the way to go.

i should have said, you can colour it the right shade first - ie: yellow for gold, and grey for silver, that will make life a bit easier, any missed/ thin spots wont be so obvious, and the colour will end up a bit stronger.
xx

Yes, brincess_b... ESPECIALLY if you color it first, it makes the gold or silver more intense.