When Raven had her heart back in the form of a red gem, after her biological heart was ripped from her chest by Trigon, she was shown to be under her father's control, viewing what he believed to be right and just.

Why did Raven turn white?

Raven has also taken on the all-white form in the comics to signify that she has been completely cleansed of her father's evil. Unfortunately, this status has not always lasted, and before long she was infected by Trigon again and turned evil once more.

What is the red thing on Raven's forehead?

Raven's Gem is a chakra-like gem on her forehead. It is a containment unit for the purpose to imprison her inner demon. When broken, her inner demon is released that can be absorbed by other demons, like Trigon.