He was of dark (Krishna) complexion and was an island born (Dwapaiyna). Hence he was called Krishna Dwapaiyana. There used to be a single Veda, he categorised the single Veda into three collections. He did a Vyas (Categorization) of Vedas and earned the monicker Veda Vyasa . The fourth Veda i.e. the Atharva Veda was recognised as a Veda much later.