Juster told Gopnik:
“I had an ailment called synesthesia,” Juster explained, pronouncing the word carefully. “I could only do numbers by colors.” His mind—in a way that will be familiar to readers of the memoirs of that fellow-synesthete [Vladimir] Nabokov—made instant, inescapable associations between a number and a color. “I can still remember a few: 4 was blue, 7 was black, and so the only way I could do math was by associating colors.”In the novel, at the Dictionopolis marketplace Milo nibbles on some letters, gaining the sensation of tastes. Later he conducts an orchestra that produces colors. And of course the whole book concretizes the abstract.
Some readers, such as Patricia Lynne Duffy in Blue Cats and Chartreuse Kittens: How Synesthetes Color Their Worlds and this article in LinguaPhile, had already recognized that Juster’s text described that mental phenomenon. But I don’t recall him speaking of his experience before.