You could hand paint it with gold lustre dust. I would check and see if you can get a can of metallic gold airbrush dye though, before I got an air brusher I used the american colours ones, and they worked really well.
There is a local cake store that sells American Colours, but i'm not sure what the Metallic Gold Airbrush Dye is though. Is that a specific brand/color?
I'm sorry, I got the name wrong, they don't call it airbrush. It's this here... http://www.amazon.com/AmeriColor-Food-Colour-Spray-Gold/dp/B009PPQ3VG
My local store sells it for less than half that amount though.