The Plastics tell Cady that "on Wednesdays we wear pink!"

 "On Wednesdays we wear pink!"

-Karen Smith

The film is dominated by the color pink. Pink can symbolize innocence and sexual purity in girls, which contrasts with “The Plastics” because they’re anything but innocent and sexually pure despite them presenting themselves to be so. Very early on, Cady learns that when you’re a “Plastic,” that you wear pink on Wednesdays. In order to sit with “The Plastics” and interact with them, you must wear pink every Wednesdays or else you have to sit with the "art freaks." 

Fun fact: Purple is the color that Feminsists attribute to Feminism.