I like to use realistic colors when I make my shells. I don't overfill, and I make sure that the air bubbles are out. I harden my shells in the refrigerator. It seems to speed up the process. I made the shells on my daughter's sweet sixteen birthday cake. I hope this helps!
I think the main thing is to use really nice quality shell molds to start with. Then use either fonadant, gumpaste, 50/50 paste or melted white or semi sweet chocolate. How you color them is up to you but I would suggest trying to keep them as close to their natural color in nature, not Sapphire blue or Emerald green, they look awful. Here are some links to wonderful shell molds.
http://www.soapymolds.com/product/sealife.html
http://www.fineartmolds.com/aquatic.html
http://www.decoratethecake.com/store/Default.asp
Luster Dust them to give the shells a realistic shine. I bought the CC luster Dust and WOW, it works great, I couldn't stop luster dusting. I bought the shell molds at 2 of the sites listed above per ShirleyW (thanks SW) and they are wonderful quality and realistic. I bought them for a wedding cake with a beach theme and they looked great. Everyone thought I bought them from a store until I told them they were chocolate. I second everything posted above too, don't overfill and tap the molds to get all of the air bubbles out, realistic colors work best.
I bought my molds at Michaels in the cake decorating area and also in the soap making area. I used the soap mold for the shells on my daughter's cake, and I used the candy mold (these were smaller) on the cupcakes for the same party. Everyone loved the shells. You can also make coral out of candy melts. I had a picture of a piece of coral. I put wax paper over it. I melted the chocolate and piped it on the wax paper following the lines. When it was dried, it peeled off the wax paper.
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