This is a halter top. A handerkerchief is a handkerchief. It's highly unlikey that this is the handerkchief in the email - HIGHLY UNLIKELY.
Ths is a typical style for little girls and young ladies for decades - DECADES. The colors are popular colors you can buy in any store.
The bandana halter top isn't anything freaky. Chances are the colors mean nothing. Other than the girls picking tehir favorite color. Although, there's always the possibility that they're like the colored wrist bands that girls wear in school indicating "what they'll do". I'm not defining DO.