Seeing Red — Dorothy P. O'Neill

A killer is prowling New York City's Lexington Avenue subway line during rush hour, stalking young women with red hair, and injecting them with a syringe full of a lethal substance. The subway murder of three redheads within two days throws the city's red-haired women into panic. Amateur sleuth Liz Rooney dyes her naturally strawberry-blond hair brown and, as usual, sets out to help her fiance, NYPD Homicide Detective Ike Eichle, solve a baffling crime. Feeling safe without her red hair to give her away, she delves into the list of suspects but eventually finds herself face-to-face with the killer in a situation where her new hair color offers her no protection.