There is much discussion about the exact meaning of the titles reference. Some say it has to do with a remark Shug makes about the color purple. Others believe it is a comment to the relationships in the book. Some think it is a reference to the bruises inflicted on Celie. Another theory references the color of Celie's room at the end of the book and how she has learned to treat herself with respect and "in theory" becoming royalty.
I would search around some more and see what I could find if I were you. There are many facinating discussions about this title.
