This book is intended for middle grade students . . . so it could be be placed in the juvenile fiction or in junior high fiction, depending how libraries, for instance, are cataloging individual collections.
I had first learned about synesthesia several years ago from the Wendy Maas book A Mango-Shaped Space. This book is more of a "boy" book than Mango-Shaped as the main character (named Basil after his mother's favorite herb!) is a boy.
It seems that the author experiences number-related synethesia, and so gave Basil her particular number/color combinations - including something that can make math more difficult than it needs to be: both three and six are shades of yellow.
Basil and his new friends Tenzie are each having tough times individually in their families. Tenzie feels that she is being completely ignored at home. Basil's long lost mother has returned for the first time in seven years. He likes his life with his grandma, but he wants to love his mother, who may prove to be just as flighty as ever.
I could not put this book down as I read it earlier this week.