"He was quite conscious of the fact that he was dark. So, he was not very outspoken."
Greatness always comes from proper utilization of anger within oneself. Rajni really did a good job at it. Besides, color of skin, Rajni had rustic emotive capability that can don villan and hero roles with equal flair unmatched anywhere-else. This could be the two main reasons for his popularity in the demographically darker inclined peninsular india. He identified his insecurities with the the insecurities of the people in tamil nadu, combined it with raw movie actor energy with immense potential, and the result is for everyone to see.