This upcoming cake I'm doing is going to be pink and I want to do the gold leaf look like this cake (this isnt what I'm doing it just shows the gold).....

http://farm6.static.flickr.com/5043/5352904244_5edb678771_z.jpg

How do I get this deep gold look? Would I let the royal icing dry then paint over it with wet luster dust? Or is there a way to mix it right in the icing before piping?

Would I let the royal icing dry then paint over it with wet luster dust? yes

Yes, that's what I would do!