you can paint on fondant with luster dust mixed with vodka or clear extract like almond. you can add a lot of dust to make it really gold or a little to just make a bit of a shimmer.
you can also add luster dust to piping gel but that wouldn't dry like it would if mixed with vodka.
as far as bc, i would use a golden or yellow paste color added to the bc and then sprinkle luster dust on top of the frosted cake, it wouldn't be as gold as the painted fondant though.
I know wilton makes spray on food coloring, i don't think they have metalics but you could try to find a gold food spray.
good luck
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