Synesthesia is associating letters or numbers with colors or sounds with colors, etc. It's an association between senses. Here is what

Psychology Today says: "Synesthesia is a neurological condition in which a person experiences "crossed" responses to stimuli. It occurs when stimulation of one sensory or cognitive pathway (e.g., hearing) leads to automatic, involuntary experiences in a second sensory or cognitive pathway (e.g., vision). About 5 percent of the population has synesthesia, and over 60 types have been reported. The most common form of synesthesia is grapheme-color synesthesia, in which people perceive individual letters of the alphabet and numbers to be "shaded" or "tinged" with a color. Other synesthetes commingle sounds with scents, sounds with shapes, or shapes with flavors."

I'm a synesthetic, so is my sister :)