Yellow, red, black and white print maybe.

Also try fuchsia. That can be a great combo, unless the cobalt and/or fuchsia are very bright. The shade matters (also with the yellow; you probably don't want to go too bright.)
Take the dress with you when shopping for the cami; it'll definitely help pick a good color/shade.

I think yellow would look great, but also don't overlook green. It really will depend on the shade of green, but I think blue and green make a beautiful combo.

Yellow immediately comes to mind. I'm trying to remember a Lucky article that put together bright colors. For some reason I am thinking cobalt and olive? I could be totally wrong. They seem to do an article on pairing bold colors every year, and cobalt is often one of them.

Like the idea of fuschia, apple green, or citron.

Thanks, girls! I'm excited to look for a colorful cami now.