Purple is the colour regarding lesbians, not so much gays or bisexuals, but it can be used as a blanket color as much as the term "gay" is a blanket term for true gays, lesbians, and bisexuals. Lesbians identify each other by this color as much as someone might say that someone wearing a rainbow ribbon is a member of the LGBT community. Blue tends to be more of a gay color, whereas pink is more a bisexual / lipstick lesbian colour. But in regards to the original question, there is no exact answer as far as I am aware of. It's like saying why is pink a "girl" color and blue a "boy" color? Why is yellow considered neutral? Someone decided that those colors represent those things, and that's the way it is - same goes for the LGBT colors and purple.