Bismillah, 
Asalamu Alaykom, 

Yes Imam Sadiq and the imams after him (peace be upon them) would of been from dark complexion as indicated by narrations. 

We even have authentic ahadith which point towards the 12th imam’s mother being dark
أخبرنا أحمد بن محمد بن سعيد ابن عقدة قال حدثنا محمد بن المفضل بن قيس بن رمانة الأشعري و سعدان بن إسحاق بن سعيد و أحمد بن الحسين بن عبد الملك و محمد بن الحسن القطواني قالوا جميعا حدثنا الحسن بن محبوب الزراد عن هشام بن سالم عن يزيد الكناسي قال سمعت أبا جعفر محمد بن علي الباقر ع يقول إن صاحب هذا الأمر فيه شبه من يوسف ابن أمة سوداء يصلح الله عز و جل له أمره في ليلة واحدة

From Yazeed al-Kunaasee said, I heard Abaa Ja`far Muhammad bin `Alee al-Baaqir (عليهالسلام), he said: “Verily, the master of this affair (i.e. al-Qaa’im) in him is resemblance of Yoosuf (عليهالسلام), he is the son of a black bondswoman (female slave), and Allaah (عَزَّوَجَلَّ) will reform his affair in one night”

Source:
1.     Al-Nu`maani, Kitaab al-Ghaybah, ch. 13, pg. 228, hadeeth # 8
2.     Al-Nu`maani, Kitaab al-Ghaybah, pg. 163, hadeeth # 3

Imam Jawad (as) according to narrations would also be insulted for his darkness and given racist abuse. These realities found in our narrations are very different to what people try to depict the imams as in drawings.

Also Sister Amina Inloes has an interesting article about this topic which may benefit you; google and download the following article entitled: 

'Racial ‘Othering’ in Shi‘i Sacred History: Jawn ibn Huwayy ‘the African Slave’, and the Ethnicities of the Twelve Imams'

May Allah grant you success