That would be handy, wouldn't it, if people who had a sexually transmitted diseases all wore a yellow handkerchief, so we'd all know? But I can't help suspecting that the practice would never be widely adhered to.

As to the girl dying her petticoats: maybe it was some pre-shrunk fabric that could be dyed, or maybe it's just poetic licence - she's saying that she might as well go around in red petticoats from now on because she's going to be thought of as a bit of a trollop.

Marje