To produce a gold and silver, or any other metallic colour:

These will need to be painted on to give the finish you require. There are several ways you can achieve this:

-  Edible lustre powder colours are generally used for dusting on, but when mixed with a clear spirit such as gin or vodka (or boiled water) and mixed to a paste they can be painted on.  These not only come in a range of golds and silvers but many other colours as well. 

- Edible gold/silver liquids.  These require diluting, and the brush cleaned with, rejuvenator spirit .  Both products give a good finish but you may have to do a second coat after the first one has dried!

- Edible Metallic Food Paints may be brushed on or if covering a large surface area, a sponge is usually used. Produced in a variety of different colours.

- Another way to achieve silver is to spray pearl lustre (5535) onto grey icing.