Angmoh just means red hair in Hokkien Chinese, who're uniformly black hair by nature. So they generalise caucasians by that term. Caucasians differentiate among themselves by colours too, e.g. brunette, redhead, blond. Angmoh as a term by itself bears no insult.
However, Chinese do insult Caucasians when there're disputes, misunderstandings or grievances. Angmoh kgow means red hair monkey. The Cantonese version is Hongmouh kguai meaning red hair demon, also abbreviated to kguai louh meaning old demon. Caucasians call each other monkeys, demons, pigs and dogs when in dispute and intending to insult.