Synesthesia is a neurologically-based condition in which stimulation of one sensory or cognitive pathway leads to automatic, involuntary experiences in a second sensory or cognitive pathway. There are some people who read letters od the alphabet and numbers and see them “shaded” or “tinged” with color. Or voices, music and assorted environmental sounds such as clattering dishes or dog barks trigger color and simple shapes that arise, move around, and then fade when the sound stimulus ends.
People who report such experiences are known as synesthetes.
This is a highly psychological topic – which is not my strength- but you can read more about Synthesia on Wikipedia.