There are many narrations that Imam Musa Al-Kadhim (AS) and Imam Ali Al-Redha (AS) were of darker complexion. (Mawsoo'at Al-Mustafa Wal Etrah by Husain Al-Shaakiri, V. 13, P.13)
Imam Al-Jawad (AS)'s mother was from Africa, from the family of Lady Maria, the mother of Ebrahim, son of the Prophet Muhammad (SAWA).
We have many narration that some of his enemies used to call him Al-Aswad (The person with darker complexion).(Tafseer Al-Ayyashi 1: 314.).
We also have narrations that Imam Ali Al-Hadi Al-Naqi (AS) was of darker complexion. (Madeenatul Ma'aajiz by Sayyed Hashim al-Bahrani, V.3, P.504.)