Hm. Foil stamping would give the desired effect, and a knock out will work but you’ll have to maintain a very tight registration. If it proves to be cost effective to do a third color run, I’ve printed an opaque white on dark background and then printed metallic ink on it with a nice effect. Not as good as foil, and you may run into trouble with the gold text disappearing on the kraft colored background if you knock out the gold areas from the purple.You can then even do another run over the gold with just varnish to make it really shiny, which I like to do. But then you’re up to 4 runs, and may just want to do the foil printing…
The other way I know how to get metallics to show up on dark backgrounds is more time consuming but VERY impressive. Do you mix your own metallics? If not, you’ll have to purchase metallic pigment/powder. Print each piece with the metallic elements in opaque white, and then immediately place the print in a shallow tray. Dip a very fine feather puff or powder puff in you metallic powder and coat the wet ink with metallic powder. It takes a bit of practice, so try it out first, but I picked this trick up from Greg Walters and it’s super dope! I wouldn’t attempt this with more than say, 500 cards or so. Also, you might want to wear a face mask if your shop is breezy. Don’t breathe the powder if it gets in the air, and keep the powder covered between prints or it will get everywhere. Good luck!