I am planning to make a 2 tiers gold buttercream with a smudges looking (not sure if that's the right term).

1. Is it better to do the crusting buttercream to make the smudge look?

2. Is it better using the gold spray ? or airbrush?

3. Will the gold color stays on the buttercream after you spray or airbrushed them?

ADo you happen to have an example to show what you mean?

Yes, something like this: 

http://styleunveiled.com/storage/blog/_national/2013/katelyn_james_styleunveiled_cake3.jpg?__SQUARESPACE_CACHEVERSION=1364423542977

Ok I made mine..by using plain white buttercream and airbrush them with yellow gel color mix with vodka since I apparently grabbed the wrong bottle. Instead of airbrush color I grabbed gel color of americolor.

So I airbrush my cake with that yellow then followed by the gold color.

The gumpaste flowers I airbrush lightly with red and gold.

AI would like to know how to do the one that was posted in the link. Would a crusting butter cream and then airbrushing it with americolor gold work? What color should the buttercream be?

AI would use a crusting BC and color it Americolor gold. Then airbrush it with Americolor gold shimmer to get it darker and give it a sparkle

I would not promise that I would match that EXACTLY. Photos can be edited, taken in a certain lighting or whatever. You never know

AGood point, thanks!