Here, Melody is at school and her classmate Claire is being really mean to her. Melody is feeling the color blue because blue represents a deep and sad feeling towards how Claire is making her feel. Melody is feeling like someone who is dumb and stupid because of her looks and abilities even though, Claire doesn't truly know who the real Melody is.
In this scene, Melody is feeling the color yellow at the end of her school's math competition. Melody is feeling this color because yellow is a bright and bold color which can be shown in a blooming sunflower or a star. This makes Melody feel like someone important, happy and special because everyone is proud of her and her team for winning the math competition.
In this last scene, Melody is at the airport where she feels the color gray. Melody is feeling gray because it represents how she is confused to why her team left without her to to the math competition in Washington, D.C. Her parents in the background are very angry for the reason that her team left her behind.