Mia lives in a world of color. Because she's a synesthete, she sees and hears colors and shapes everywhere, especially when listening to music. And because of this, the book is filled with passages like this one, especially when Mia's alone:
The colors immediately and gently flow over me, energizing me, reminding me that I can still enjoy them. The glossy red-barnlike color of the violin, the silvery-bluish white of the flute, the school-bus yellow of the French horn. All of them layering on top of one another, changing, shifting, belonging, at that minute, only to me. (1.73)
Notice the way Mia describes the color as "belonging"to her. It's not just that she sees colors when others don't, but that she feels a sense of ownership over these colors. In her room, painting and listening to music, Mia is free to be who she really is. After keeping her synesthesia a secret for so long, this time in her room is Mia's chance to be her full self. Fittingly, then, the colors that she sees are vibrant and beautiful, mirroring the fact that in moments like this Mia is finally free.