Super ages ago I had the idea for these (though just a tiny amount of googling informs me that I was not the first), and a Mermaid Party was JUST the excuse to make them. I *almost* forgot them entirely, and remembered only recently, after some beach combing. Whew.
They were a little time-consuming, but def worth the effort. I think I used about four coats of paints, and probably couldn't done another coat or two. I had ten to make, and I ditched early. I think they look fine *crosses fingers*. I also wish I'd glued the 'pearl' after painting the shell. I made the call to glue first, hoping that I could tidily paint around the pearl and disguise the glue. It didn't work out marvellously, but I ended up kinda blending the pearl into the shell with white paint, so I guess that's okay.
My original idea included iridescent paint, but that stuff is expense! So I used what I had on hand, and a little bit of glitter looked just as good. A fun, simple idea that was easy to execute. Perfect.
Edit - The placement of the pearl is not especially accurate - but we wanted the pearl as a placeholder for rings 'n things. Yanno. Creative licence and all that. I mean, it's a scallop shell so we're not going for complete accuracy here ;-)