Maybe I looked at the wrong Union waterbased gold , the only one I can find is the Union Aerotex shimmer gold. Or is there another waterbased gold available from Union?
I don't have experience with the Union WB inks, but most metallic WB inks have the opacity to P/F/P and lay down a nice print on dark cotton shirts. If you get a batch thats a bit runny and you have some time, leave the bucket without a lid for a few days and allow some of the water to evaporate. The less water, the more opaque they'll print out of the bucket, but as a side effect they also become a bit tougher to work with.
In regards to the Matsui inks, its really not too tough to mix a gold from the silver, and I can send you our formulas if you decide to go that route. They are the PC colors though, and I understand that Matsui has discontinued that line. But you should be able to get close with any system using our formulas as a guide. We've done bronze, copper, pink, purple and all sorts of other metallics using that ink and its no more difficult than mixing a standard color.