What is the color of the hat?

The only situation that Alice can tell what color she was wearing was that nobody was wearing blue hat. In this situation, she must had wearing a blue hat. We can be sure one of the girls in front of Alice was wearing blue hat. Betty knew the above statement was true. If neither Cindy nor Debbie was not wearing blue hat, then she can be sure she was wearing blue hat. Obviously she saw a blue hat in front of her. This was why she could not tell. Cindy had the same situation as Betty. She cold not tell because she saw a blue hat in front of her.
Debbie knew all the above facts. So she told Mrs. Harrison that she was wearing a blue hat.