Love
Elphaba still had trouble accepting the fact Fiyero loved her, green skin and all.
It wasn't that she didn't believe him, she simply didn't understand.
After years of being shunned for her verdigris she'd come to accept the taunts of others as truth. Fiyero's claims went against all she'd ever known.
Admittedly, she gained some perspective in light of her spell gone wrong. Her love for Fiyero had not changed, regardless of his appearance. Still, she had difficulty applying the concept to herself.
When she first held her newborn child - tiny, innocent, beautiful, and green - she finally understood.