For a 1920s-themed costume, white braces are the ultimate "high-society" or gangster accessory.
Formal Wear (or Great Gatsby) - over a white shirt, these braces don't stand out but still give a time period accurate look
Gangster -White braces over a pinstripe or solid dark shirt (navy, black, or burgundy) create a sharp, graphic "tough guy" look.
 
Pair with a white tie and scarf.