why was this novel titled the color purple

The best expplination that I can help you with is right out of this site.

"Think about when purple is mentioned as a color and as an attribute of something worth admiring. Should all colors be admired equally, all being features of things created by God? Does rarity really make something more precious--purple seems rare in nature, but does this make other colors any more or less valuable? Consider the same question with regard to racial majorities and minorities in Africa and in the United States, which also are often distinguished by color."

Please look at the source link below to GradSaver for more detail.