I need some advice on how to make edible seashells look the most realistic. I have a destination wedding cake coming up soon and it has seashells all over it and I want them to look as real as i can possible get them. I don't have an airbrush though

I've made seashells out of white chocolate in a mold then dusted them in luster dusts using a small paintbrush. You can use whatever color you want, and match their colors or to look realistic.

There are some awesome life like molds out there for shells.

I have made them using fondant/gumpaste and dusting them with pearl, petal and luster dusts in various different shades.

Definitely do them in white chocolate or modeling chocolate. People love them. They are very beautiful dusted with the pearl shimmer dust.

Here's another vote for white choco
Here are a few of those ones I have made : http://cakecentral.com/gallery/1331641/seashell-trio
http://cakecentral.com/gallery/1300406/tropical-sunset
http://cakecentral.com/gallery/1294591/underwater
http://cakecentral.com/gallery/84663/romantic-wedding-shower-design

I used gumpaste and placed in the mold, them in the freezer then took out, while it was a bit tacky I used a mixture if petal dust and luster dusts. Everyone thought they were real