If you are looking for a way to distinguish your frags... maybe try a different SHAPE plug as no matter what grows over... the original plug will be seen under it.... maybe try a star shape or something for example.

Or how about imprinting the disks/plugs rather than coloring? A person could probably use something like macaroni to imprint a shape and then remove it later while it soaks and cures.

+1. Or if you are looking at using something metal for the mold, you could always stamp the metal so it molds into the frag plug.