“[T]he most commonly reported type of synesthesia is hearing or seeing letters in color. Vowels, and often consonants, too, have very specific—and fixed—colors for the synesthetes who see letters in color. For the synesthete Katinka Regtien, for example, the vowel E is not simply red but a specific translucent red with a hint of orange. . . .
For the synesthetes who see colored letters, the colors normally remain the same throughout their lifetime, though older synesthetes recount that the colors sometimes become paler in their later years. . . . The colors are so obvious to them that young synesthetes believe that everyone sees letters in color; many synesthetes only discover later that this is not the case.”
—Cretian van Campen, The Hidden Sense: Synesthesia in Art and Science, 2007.