Calling all icing/frosting coloring experts! I would like to use a product to color buttercream in a shimmering gold. What is the best way to get a shiny gold tone for the frosting?

you apply shiny stuff after you apply the icing to the cake -- you can color the icing a yellowy goldish color to enhance the shiny stuff -- you'll need edible color for the shiny stuff -- you can paint on wet paint that you mix up with everclear or vodka or lemon extract -- and you could also brush the dry powder onto a cold cake --

https://www.globalsugarart.com/ssearch?q=gold%20luster%20dust

I painted luster dust/vodka mix on cold buttercream and it stayed pretty shiny! You would just need a lot more and a biggest brush to paint the whole thing!