This clip shows Joy explaining core memories and how they work to create Riley's personality.

“Core memories” are a device used in the film to explain how major moments in our lives shape our personalities. Each core memory powers an island of Riley’s personality and is vital in making her who she is. When she is young, Riley has 5 personality islands; family, friendship, goofball, hockey, and honesty islands. By the time the film ends, Riley is 12 and she now has dozens of personality islands including tragic vampire romance island, boy band island, and an improved friendship island with a friendly arguments section. 

All of Riley’s core memories up until her first day of school were yellow because they were happy memories. After Sadness comes along and touches Riley’s memories of Minnesota, they turn blue and cannot be turned back. When Riley returns home from trying to run away and is comforted by her parents, she creates her first core memory that is both happy and sad. The new core memory powers a larger family island than the one she previously had and includes aspects of both San Fransisco and Minnesota. Thereafter, all of Riley’s core memories have aspects of multiple emotions rather than just joy.

A. O. Scott argues that Joy trying to make Sadness stand in a circle so she doesn't touch anything or make any more memories blue is “a pretty powerful metaphor for repression,” which he says if often imposed on children by well-meaning parents who just want their kids to put on a happy face. This film shows, however, that sadness (and all of our other emotions) are an important part of forming memories and making us who we are. 

As previously discussed, Riley was used as a gender-neutral character in order for her experiences to be more relatable to those of all audience members. Once she turns 12, however, Riley begins to have aspects of her personality that are more "typical" of a young girl, such as boy band island and fashion island. This is a device used to show how kids come of age and some choose to become more feminine or more masculine.