Melisandre has a good and evil view of the world. She moralizes between believers and non-believers of R’hllor.
Beneath the weeping Wall, Lady Melisandre raised her pale white hands. “We all must choose,” she proclaimed. “Man or woman, young or old, lord or peasant, our choices are the same.” Her voice made Jon Snow think of anise and nutmeg and cloves. She stood at the king’s side on a wooden scaffold raised above the pit. “We choose light or we choose darkness. We choose good or we choose evil. We choose the true god or the false.” (ADWD Jon III)
Melisandre’s visions are always completely true in some interpretation. However, she is cursed not to observe anything that would confirm that they are true. Particularly when she’s trying to prove the legitimacy of her visions to another person. The evidence of their truth is often obscured from her point-of-view. This tragically causes her to doubt herself and to begin to consider herself a failure.
Examples: Renly does indeed fight at Blackwater, Arya is indeed the Girl in Grey
Apples and Dragons 