When I make marbled fondant or candy clay I always end up overmixing it and the colors don't look distinct enough. By using this layering technique to make the candy clay for the shells you can avoid the overmixing and keep the colors of the shells distinct.
Here's the video:
The link to the shell molds is here: Seashell molds
Kara Buntin owns A Cake To Remember LLC, online cake supplies at www.acaketoremember.com and www.acaketoremember.etsy.com