Theoretically, they should never be seen, so you could wear neon green if you're not taking your jacket off. To answer your question, according to TPTB, black or white silk, or a combination of the two, are appropriate choices.
daniel craig rocks the white braces and black bowtie well in casino royale. i'm a fan of matching the bowtie, but poster above is correct... you're never actually supposed to see someone's braces, so wear what you think looks good.
How about if a gentleman needs to remove his jacket in order to defend a lady's honor? Either color works though, but I would stay away from neon green.
