One + One = Blue

Twelve year-old Basil knows he's special―he's been associating numbers with colors since he was a kid. His gift (or curse) has turned him into somewhat of a loner, but his world begins to change when he meets Tenzie, the new girl in school who has similar freakisms. She, too, has synesthesia (a condition in which one type of stimulation evokes the sensation of another). At first, Basil is somewhat annoyed with Tenzie's pushiness, but after Basil's estranged mother returns, his life is turned upside down . . . and Tenzie may be the only person to help him put it back together again.
Once again, MJ Auch has written a thoughtful coming-of-age novel that explores friendship, family, and fitting in, in One Plus One Equals Blue.

From School Library Journal

Gr 5-8-Basil has just started seventh grade after being homeschooled by his hippie-era grandmother. At first he thinks he wants to make friends with other students, but he soon decides that he is just too freaky and different to ever have any friends. In October, when Tenzie shows up at school, everything changes. She is pushy and determined to befriend Basil, whether he likes it or not. When he finds out that Tenzie sees numbers as colors, too, he is prompted to do some research. He discovers that they both have the same neurological condition, only Tenzie's synesthesia helps her with math, whereas Basil's makes him hopelessly confused. Life gets topsy-turvy when Basil's mother, who abandoned him seven years earlier, shows up in town. Basil is wary of Carly, but Tenzie is enamored-the woman is beautiful, glamorous, and claims to be an actress. When she abruptly leaves town once again, Tenzie convinces Basil to run away with her and find Carly. The kids go on a harrowing journey only to discover that everything they need is back home. Synesthesia is an important bond between Basil and Tenzie, and readers are led to believe that the condition is going to be more central to the plot, but this is primarily an engaging story of a boy coming to terms with the shortcomings of his mother. It's a nice companion to Wendy Mass's A Mango-Shaped Space (Little, Brown, 2003), which also incorporates synesthesia.-Ragan O'Malley, Saint Ann's School, Brooklyn, NY