Every susano o had a different color this one is exception. Since madara and sasuke's are both a dark color i think its because they both received their power through losing their brothers who they loved more than anything. Itachi got his protecting sasuke by killing his parents which i thick is what gave him less of a dark look like madara or sasuke. So considering Indra is doing this out of sheer hate towards his brother it might be some other dark shade cant really say which though i jus hope its not black that would be way to predictable.