Puzzle: What is the color of the hat?

Mrs. Harrison has 6 lovely hats, 3 blue, 2 yellow and one pink. Alice, Betty, Cindy and Debbie are lined up as shown in the figure. Mrs. Harrison help them put the hats on them so they will not see what color hat they have on.

Alice can see what color of hats Betty, Cindy and Debbie are wearing.
Betty can see what color of hats Cindy and Debbie are wearing.
Cindy can see what color hat Debbie is wearing.
Debbie can not see any of the hat colors.

Mrs. Harrison ask them what color of the hat they are wearing. Alice said she can not tell. Betty said she can not tell either. Cindy also can not tell. However, Debbie was able to tell what color she was wearing after knowing that everyone else could not tell.

How did Debbie figure out what color she was wearing?