Why did Norah choose to dress so vividly after her divorce? What was she trying to convey?

Back when Derek was bedridden in a vegetative state, Norah didn't hesitate to marry him and look after him with excellent care, showing just how much she loved him with all her heart.

Even after Madeline came back, Derek still held onto his feelings for her. He decided to leave Norah and be with Madeline. While he stuck to his choice, there were moments when he wished he hadn't.