Synesthesia is one of the most fascinating phenomena. In its simplest example, one hears a sound and sees a color associated with that sound. Stevie Wonder has stated that, even though he’s blind, his music is specifically filled with colors he sees in his mind depending on the key and rhythm of the sound: Visual Hearing. In its more sophisticated form, Synesthesia changes the colors of words or numbers in a sequence depending on the context of arrangement.
For example, an outline of the number 7 might be perceived as being “seven” and trigger the color blue, but when you look closer — and learn that the 7 is actually made up of many tiny number threes — then the color changes from a 7 trigger color to a 3 trigger color like, say, gold. Some synesthetes can actually taste the colors as they view them.